The issue regarding INJ_MAIN was due to the read of problematic post-processing channels found in the full frame and possibly linked to the mix of Frame library versions in use.
The solution about the new Metatron version using the fixed Frame library will be pursued during the move to Python3; for the time being it was sufficient to use the FDIN_TAG facility (see #55487) in order to acquire from the frame only the needed and standard channels from the DAQ.
INJ_MAIN was then restarted correctly and ITF_LOCK, which was temporarily disconnected from its use, has been restored correctly.
Right now, the only nodes which still are missing the FDIN_TAG functionality are SQZ_MAIN, SQZ_FLT, TCS_MAIN and CALI.
The TCS_MAIN Metatron node has been updated to use the FDIN_TAG functionality; possible issues will be addressed when the node will be in use again.
BRMSMon recompiled against Fd v8r43p3, as suggested by Benoît. New tag created (v6r0), VPM configuration updated and server restarted.
The problem was that the old olserver52 server hardware has been turned on by mistake and therefore it was causing IP address and authentication membership contention with the current server.
The good news are that there are no native failures in the current olserver52 server and it can be used again in redundancy with olserver53