Dovecot/Postfix Error: connect failed to database

I have a question and I hope you can help. I have an email server setup and running. A few days ago the following updates were installed:

2019-08-15 22:23:40 upgrade php-common:all 2:69+0~20190303094804.15+stretch~1.gbp0f7465 2:70+0~20190814.17+debian9~1.gbp1e7da2

2019-08-15 22:23:44 upgrade php-igbinary:amd64 3.0.1+2.0.8-1+0~20190503122633.10+stretch~1.gbp63e2f2 3.0.1+2.0.8-2+0~20190814.12+debian9~1.gbpaafd11

2019-08-15 22:23:46 upgrade php-imagick:amd64 3.4.4-1+0~20190808.10+debian9~1.gbpc5da26 3.4.4-1+0~20190814.12+debian9~1.gbpc5da26

2019-08-15 22:23:48 upgrade php-msgpack:amd64 2.0.3+0.5.7-1+0~20190220080019.9+stretch~1.gbp75b3fa 2.0.3+0.5.7-2+0~20190814.11+debian9~1.gbpb26058

2019-08-15 22:23:49 upgrade php-memcached:amd64 3.1.3+2.2.0-1+0~20190606080312.9+stretch~1.gbpb26597 3.1.3+2.2.0-2+0~20190814.14+debian9~1.gbp5d60d1

2019-08-15 22:23:51 upgrade tzdata:all 2019a-0+deb9u1 2019b-0+deb9u1

2019-08-15 22:23:53 upgrade php:all 2:7.3+69+0~20190303094804.15+stretch~1.gbp0f7465 2:7.3+70+0~20190814.17+debian9~1.gbp1e7da2

2019-08-15 22:23:54 upgrade php-mysql:all 2:7.3+69+0~20190303094804.15+stretch~1.gbp0f7465 2:7.3+70+0~20190814.17+debian9~1.gbp1e7da2

After these updates I realized I had trouble accessing email, calendar and contacts, which are synchronized through z-push (ActiveSync) to any of my devices. Before these updates everything was working fine. I uninstalled PHP completely from the system and reinstalled it again, as suggested in the post here on the Nextcloud forum. The updated system is only running for a few minutes now but I ran into the same issue again. dovecot: auth-worker(16752): Error: mysql(127.0.0.1): Connect failed to database (postfix): Can't connect to MySQL server on '127.0.0.1' (111 "Connection refused") - waiting for 1 seconds before retry

My database processes between 1,000 and 5,000 queries a second, which, if I read my logs right, stem mostly from Nextcloud.

I would appreciate any feedback on how I can optimize my system so my email functionality is not impacted by any activity going on in the database.