FTPS Connection - TLS session resumption

I am fighting kind of an anomaly here.
I recently checked the duplicati gui saw that there where no backups done in 13 days.
The reason beeing Error 450, and then some mumble jumble where I’m not sure it was correctly translated.
But because I keep my filezilla server logs I can tell this much:
“450 TLS session of data connection has not resumed or the session does not match the control connection”

I did neither change something in Filezilla Server, nor in Duplicati.
None of the above did receive an update.
Nonetheless I now have do uncheck “Require TLS Session resumption on data connection when using PROT P”.
Which is both an potential security risk and a unnessesary performance overhead.

Does anyone have an idea how that happened?

Edit: I have done some further checking:
This issue persists with another installation of duplicati as well.
I also used the duplicati backup of the server machine to examine my FileZillaServer.xml, several probes between everything was set up, nearly a year ago, and the day stuff started failing, two weeks ago.
This Line remained unchanged:
< Item name=“Force TLS session resumption” type=“numeric”>1< /Item>
(Two spaces added so the forum doesn’t html the crap out of that line)

This leaves me just amazed.

Edit Number 2:
Now I feel sorry for harrassing this forum first, but afterwards i wanted to check if maybe filezilla server is the problem, and voila:
I’ll leave the post up though so that it might save someone an hour at least worrying about their sanity, like it did in my case.