Hello,
I have same problem. In my case, server 2.5 upgraded 2.6 nightly 2/2/2020. I solved the problem, but i don't know excatly how
1/ i upgraded server with 2.6 nightly 8/2/2020
2/ before last upgrade, deleted plugins directly on the web interfaces. deleted some old files
in /usr/share/ocsinventory-reports/ocsreports/extensions
/etc/ocsinventory-server/plugins (old v2.5)
/etc/ocsinventory-server/perl/Apache/Ocsinventory/Plugins (old v2.5)
3/ install correctly plugins :
hope help.
Regards.
PS : Sorry for my very bad english and format text (copy/paste from word ^^)