I was a bit ahead of you and took the liberty to install a completely new system: raspbian buster which contains mono 18.104.22.1689. That did not improve things, on the contrary.
I removed 22.214.171.1249 and installed 126.96.36.199 according to your 5.x suggestion (which was NOT a singular operation).
That did NOT solve the problem.
So what I left with is reverting some old system. I had one backup image from a week ago, just before the problem. I'll put that back and copy the current Cumulus install over it. See what that does. I'll get back on this (currently restoring).
NB: During the whole process of trial and error, I got several times the following message:
Maybe coincidence, maybe related.Instellen van monodoc-http (4.2-3xamarin1+raspbian10b1) ...
Use of uninitialized value $libs in concatenation (.) or string at /usr/sbin/mono-xsp4-update line 216.
[ ok ] Restarting mono-xsp4 (via systemctl): mono-xsp4.service.
It might be interesting to contact the development group of Mono and notify them of this issue.
You can't stay on Jessie or Stretch forever, the world turns.
If you don't and the problem does not magically go away in subsequent versions, you may have to solve it in CumulusMX itself, dates and strings are too important in Cumulus.
I will follow up with the result of my restore exercise.
After that I will stop this,it takes too much time.