"We have uninstalled the blog plugins" ... the core blog plugin or
an addon blog plugin ... or both?
3.5 is now 3.5.7.
Moodle 3.5.6 13 May 2019 and was the last point release
in that version for bug fixes in code: Bug fixes for general core bugs in 3.5.x ended May 2019 (12 months)
If you have a clone of production server that is set up just like production, suggest updating the clone to 3.5.6 first then see if the issue persist.
That update on your dev box (the clone) will probably restore the core blog plugin deleted. But that's OK. Just hide core plugins. if you do not want to use them (don't delete).
If programmers of moodle were still fixing 3.5.x you might have discovered a conditional bug (only happens when deleting core blog), but ... even then, response to tracker posting might be ... not a bug ... with recommendation to just hide blog plugin.
There was no upgrade to php at the same time, was there? If so, what version of PHP?
'SoS', Ken