Thank you for your hint.
I’ve tested a bit with the different options and values and came to the conclusion, that already
AmbientCapabilities=CAP_DAC_OVERRIDE
solves the problem. So the rest of the options could be left with default settings.
So it could be a permission problem when starting freeradius via systemd.
During various service-restarts I noticed the following messages:
Mär 17 12:08:51 hostname freeradius[2606397]: Configuration appears to be OK
Mär 17 12:08:51 hostname freeradius[2606398]: ber_get_next failed, errno=11.
Mär 17 12:08:51 hostname freeradius[2606398]: ber_get_next failed, errno=11.
Mär 17 12:08:51 hostname freeradius[2606398]: ber_get_next failed, errno=11.
Mär 17 12:08:51 hostname freeradius[2606398]: ber_get_next failed, errno=11.
Mär 17 12:08:51 hostname freeradius[2606398]: ber_get_next failed, errno=11.
Mär 17 12:08:51 hostname systemd[1]: Started freeradius.service - FreeRADIUS multi-protocol policy server.
and
Unit uses MemoryLimit=; please use MemoryMax= instead. Support for MemoryLimit= will be removed soon.
The first one showed up even if freeradius was started manually. The seconed one is more of an information about an upcoming change.
So both of them shouldn’t have to do with the problem itself.