Local services state UNKNOWN after upgrade


after upgrading a recently inherited Icinga 1 installation to Debian Buster / Icinga 1.14.2 (which i know is outdated) i run into the following problems:
13 of 16 local, active checks on the icinga host occasionally have status UNKNOWN, the state is flapping. The relevant service are: sys-clock, sys-linux-cpu, sys-linux-disk-free, sys-linux-disk-io-bytes, sys-linux-disk-io-reqs, sys-linux-load, sys-linux-mem, sys-linux-net, sys-linux-nfiles, sys-linux-paging, sys-linux-procs, sys-linux-sockets.
What i did so far: looked into log files, raised the debug level, checked file attributes, disabled Apparmor, rebooted. So far no luck.
Any hints what i can check?

Best regards,