reason was part of the installation of the DR CommServe (using CommServe LiveSync).
I forgot to install SNMP Enabler on the DR CommServe.
after I failed over to the other node and switched backup to the original, SNMP Enabler couldn't be used any longer on the production node, even though the software was installed.
I failed over to the DR CommServe again, installed SNMP Enabler there as well, while CommServe was active on that node and failed backup to production again.
everything is working now.