Backups failing due to access problems

We're using Veritas Backup Exec 16.0 to run backups of our servers. We've upgraded from Backup Exec 2010 and have been firefighting problems since then trying to get the schedules to run smoothly. Veritas Support have generally be utterly useless but have resolved some problems. One of the ongoing problems is backups failing on certain servers.
We've created a Active Directory service account called backupexec and set this as the sole Backup Exec 'System Logon Account' for running backups. This account is part of the AD 'Domain Admins' and 'Backup Operators' groups.
We have two email servers; we'll call them MAIL1 and MAIL2. MAIL1 is the main email server and MAIL2 is for email archiving. There are directories on both servers which we want to back up. All of the archives have read/write permissions set for the backupexec account but we have tried with full control set as well.
Jobs in Backup Exec are failing with errors such as "The directory or file was not found, or could not be accessed." I have checked both servers and the directories are indeed there and can be accessed by the backupexec account.

I had thought this may be a Backup Exec problem, given all the other problems we're having with the software but I have since run a Windows Server Backup of the same directories. Both have failed with "Backup failed to complete - The device is not ready" errors. I suspect that this may be related to Volume Shadow copy Service problems but do not know why this would be failing.
Inspection of the Event Viewer (Application and Services Logs > Microsoft > Windows > Backup > Operational) shows an error event with ID 5 "The backup operation that started at '‎2018‎-‎05‎-‎15T13:06:43.666836500Z' has failed with following error code '2155348238'. Please review the event details for a solution, and then rerun the backup operation once the issue is resolved." Searching the web for this error code does not reveal too much so I'm posting here to see if anyone has had anything similar or can offer any advice.

Checking the VSS writers and providers does not show any problems:

MAIL1

Microsoft Windows [Version 6.1.7601]
Copyright (c) 2009 Microsoft Corporation.  All rights reserved.

C:\Users\*****>vssadmin list providers
vssadmin 1.1 - Volume Shadow Copy Service administrative command-line tool
(C) Copyright 2001-2005 Microsoft Corp.

Provider name: 'Microsoft Software Shadow Copy provider 1.0'
   Provider type: System
   Provider Id: {b5946137-7b9f-4925-af80-51abd60b20d5}
   Version: 1.0.0.7

Provider name: 'Acronis VSS SW Provider'
   Provider type: Software
   Provider Id: {f782463b-33bb-4043-ad8d-60b728d26a6c}
   Version: 1.0


C:\Users\*****>vssadmin list writers
vssadmin 1.1 - Volume Shadow Copy Service administrative command-line tool
(C) Copyright 2001-2005 Microsoft Corp.

Writer name: 'Task Scheduler Writer'
   Writer Id: {d61d61c8-d73a-4eee-8cdd-f6f9786b7124}
   Writer Instance Id: {1bddd48e-5052-49db-9b07-b96f96727e6b}
   State: [1] Stable
   Last error: No error

Writer name: 'VSS Metadata Store Writer'
   Writer Id: {75dfb225-e2e4-4d39-9ac9-ffaff65ddf06}
   Writer Instance Id: {088e7a7d-09a8-4cc6-a609-ad90e75ddc93}
   State: [1] Stable
   Last error: No error

Writer name: 'Performance Counters Writer'
   Writer Id: {0bada1de-01a9-4625-8278-69e735f39dd2}
   Writer Instance Id: {f0086dda-9efc-47c5-8eb6-a944c3d09381}
   State: [1] Stable
   Last error: No error

Writer name: 'System Writer'
   Writer Id: {e8132975-6f93-4464-a53e-1050253ae220}
   Writer Instance Id: {38a136f6-947f-4cc1-858d-7da11af035b1}
   State: [1] Stable
   Last error: No error

Writer name: 'ASR Writer'
   Writer Id: {be000cbe-11fe-4426-9c58-531aa6355fc4}
   Writer Instance Id: {775938d0-040d-439e-b23c-470caa2f4c84}
   State: [1] Stable
   Last error: No error

Writer name: 'Registry Writer'
   Writer Id: {afbab4a2-367d-4d15-a586-71dbb18f8485}
   Writer Instance Id: {8dd82e55-494f-4a0a-85d0-feda8087da0e}
   State: [1] Stable
   Last error: No error

Writer name: 'Shadow Copy Optimization Writer'
   Writer Id: {4dc3bdd4-ab48-4d07-adb0-3bee2926fd7f}
   Writer Instance Id: {062648bc-5c63-4480-95c7-060c7ffa81ff}
   State: [1] Stable
   Last error: No error

Writer name: 'COM+ REGDB Writer'
   Writer Id: {542da469-d3e1-473c-9f4f-7847f01fc64f}
   Writer Instance Id: {7f98ce15-383e-45d0-8882-e40ae9d573a9}
   State: [1] Stable
   Last error: No error

Writer name: 'BITS Writer'
   Writer Id: {4969d978-be47-48b0-b100-f328f07ac1e0}
   Writer Instance Id: {546b3f33-5754-45e8-aa35-199fb87e5239}
   State: [1] Stable
   Last error: No error

Writer name: 'WMI Writer'
   Writer Id: {a6ad56c2-b509-4e6c-bb19-49d8f43532f0}
   Writer Instance Id: {e282e3a3-cc1e-4848-831c-fc90bb230a6a}
   State: [1] Stable
   Last error: No error

MAIL2

Microsoft Windows [Version 6.1.7601]
Copyright (c) 2009 Microsoft Corporation.  All rights reserved.

C:\Users\*****>vssadmin list writers
vssadmin 1.1 - Volume Shadow Copy Service administrative command-line tool
(C) Copyright 2001-2005 Microsoft Corp.

Writer name: 'Task Scheduler Writer'
   Writer Id: {d61d61c8-d73a-4eee-8cdd-f6f9786b7124}
   Writer Instance Id: {1bddd48e-5052-49db-9b07-b96f96727e6b}
   State: [1] Stable
   Last error: No error

Writer name: 'VSS Metadata Store Writer'
   Writer Id: {75dfb225-e2e4-4d39-9ac9-ffaff65ddf06}
   Writer Instance Id: {088e7a7d-09a8-4cc6-a609-ad90e75ddc93}
   State: [1] Stable
   Last error: No error

Writer name: 'Performance Counters Writer'
   Writer Id: {0bada1de-01a9-4625-8278-69e735f39dd2}
   Writer Instance Id: {f0086dda-9efc-47c5-8eb6-a944c3d09381}
   State: [1] Stable
   Last error: No error

Writer name: 'ASR Writer'
   Writer Id: {be000cbe-11fe-4426-9c58-531aa6355fc4}
   Writer Instance Id: {53545b61-ebbb-46b9-8e4d-0bf0e3b542f6}
   State: [1] Stable
   Last error: No error

Writer name: 'WMI Writer'
   Writer Id: {a6ad56c2-b509-4e6c-bb19-49d8f43532f0}
   Writer Instance Id: {24a63d31-b3e2-42a9-9406-8ee5199d5155}
   State: [1] Stable
   Last error: No error

Writer name: 'System Writer'
   Writer Id: {e8132975-6f93-4464-a53e-1050253ae220}
   Writer Instance Id: {a39861ce-cd1f-48f3-a5f7-1f64030e6c49}
   State: [1] Stable
   Last error: No error

Writer name: 'Shadow Copy Optimization Writer'
   Writer Id: {4dc3bdd4-ab48-4d07-adb0-3bee2926fd7f}
   Writer Instance Id: {2860109e-f5e6-483c-b3be-74610f7d33f2}
   State: [1] Stable
   Last error: No error

Writer name: 'COM+ REGDB Writer'
   Writer Id: {542da469-d3e1-473c-9f4f-7847f01fc64f}
   Writer Instance Id: {2335da7a-8a8d-4cc1-b9fb-5330228379cb}
   State: [1] Stable
   Last error: No error

Writer name: 'Registry Writer'
   Writer Id: {afbab4a2-367d-4d15-a586-71dbb18f8485}
   Writer Instance Id: {54bb9048-bb0e-4fca-bd13-c336add2d72a}
   State: [1] Stable
   Last error: No error


C:\Users\*****>vssadmin list providers
vssadmin 1.1 - Volume Shadow Copy Service administrative command-line tool
(C) Copyright 2001-2005 Microsoft Corp.

Provider name: 'Microsoft Software Shadow Copy provider 1.0'
   Provider type: System
   Provider Id: {b5946137-7b9f-4925-af80-51abd60b20d5}
   Version: 1.0.0.7

Provider name: 'Acronis VSS SW Provider'
   Provider type: Software
   Provider Id: {f782463b-33bb-4043-ad8d-60b728d26a6c}
   Version: 1.0

I'm not sure what else could be preventing access.

I'm currently running another backup on MAIL2 using my admin account (also a member of Domain Admins) instead of the backupexec account to see if this makes any difference. I'll post the results when they're in.

Thanks in advance.