Sorry it took so long to respond, I’ve been unavailable for a while. Thank you so much for your assistance so far.
As expected, cmk -vvR with removed ~/local/share/check_mk/checks directory has no effect, since all the error-inducing checks are builtin.
A new check_mk site with the same version does not have these symptoms (i.e. cmk -vvR does not yield any negative results). The only difference is that the new site is fresh and hasn’t undergone the 1.6→2.0 update.
Interestingly, a direct „site-by-site“ comparison (pun intended) of the files named after one check (fast_lta_volumes) yields the following:
root@monitoring ~ # md5sum ~monitoring{,_clean}/tmp/check_mk/check_includes/builtin/fast_lta_volumes
68b329da9893e34099c7d8ad5cb9c940 /omd/sites/monitoring/tmp/check_mk/check_includes/builtin/fast_lta_volumes
68b329da9893e34099c7d8ad5cb9c940 /omd/sites/monitoring_clean/tmp/check_mk/check_includes/builtin/fast_lta_volumes
But also
130 root@monitoring ~ # md5sum ~monitoring{,_clean}/var/check_mk/precompiled_checks/builtin/fast_lta_volumes :(
9faf94c307530d4d698d82c786d624b4 /omd/sites/monitoring/var/check_mk/precompiled_checks/builtin/fast_lta_volumes
c65d649da472524faff05058bbdbaa11 /omd/sites/monitoring_clean/var/check_mk/precompiled_checks/builtin/fast_lta_volumes
i.e., the checks seem to be the same, but the precompiled checks differ.
Perhaps I should try removing the precompiled_checks folder?