I've downgraded to a system with kernel 4.19.63 as mentioned in the posting by Mangix above. Since then the systems runs without freezes (already for a few weeks).

 

Two things should be noted:

- The "blinkenlights" - i.e. the hard drive access LEDs - do work again. They didn't work at 5.8.16.
- The CPU load of the netatalk filesystem daemon is now significantly lower. This would be consistent with the remark from gprovost that my error message from the earlier system points to a system overload. I do not know if the now lower load is due to the older kernel or due to the older version of netatalk running now.

@karamikeThe issue has been found and fixed. You should update to a more recent kernel (latest legacy or current kernel). I'm on 5.9.14 right now. Works flawlessly.

 

 

More info on DFS brokenness: https://gitlab.nic.cz/turris/openwrt/-/issues/347#note_195030

DVFS patch were removed quite some time ago in Armbian since it wasn't stable and the gain in term of power consumption & thermal was negligible.

Yeah I know. I removed them. Simply posting for more info.