LibreOffice 7.1 Writer flickers and crashes

I am using Libreoffice 7.1.4.2 under Ubuntu-MATE 21.04. According to the LO's about dialog: User Interface: UI render: default; VCL: gtk3

I only use the text processing „Writer“, nothing else.

Problem 1: my styles panel (on the right) on the right flickers all the time

...blinks, maybe 50-100ms to white and back... (but certainly not in a blink tag fashion, more of a “nervous redraw”)

• basically irregulary about once per second (1 Hz)
• more often with every keypresses happening
• not at all, if focus is on another application's windows
• only in the styles panel

So apparently triggered by some refresh mechanism.

Problem 2

I assume it's related, thus in one post. LibreOffice freezes my entire machine:

• mouse still movable, all screens showing (3 in total, including 2× 4K)
• that's all – no clicks, no hovers, no interaction
• no keyboard commands (i.e. for Run or Terminal or alt-Tab) working
• not even ctrl-alt F3, F6, F7 (for non-graphical login)
• only hard reset + reboot helps

Any ideas?

What log file to look into? How to put LibreOffice into a different rendering mode?