Certainly not a helpful error. It could be a bug if you said that is worked fine in an older version. Did your reseller forward your request to Safe Software?

No, not yet. They asked if I checked this page: https://support.safe.com/hc/en-us/articles/25407577604365-FME-Flow-Troubleshooting-Version-Control
But that was not helpful.
I can commit, just not push. The SSL error doesn't seem to be the problem, but to be sure I added the certificate of https://dev.azure.com/ to the store. That didn't help.
Now I'm waiting for their reply.

tl;dr: Before the update FME Flow created the "master" branch and did everything in that branch. 
After the update it seems that FME Flow has checked out a branch called "main" and created it in the remote repository. But somehow FME Flow is expecting the "master" branch.
I don't know exactly what I did, because my git-skills are quite limited, but I checked out the master branch on the FME Flow-server and now it works fine.  

I do think it is some kind of bug in FME Flow.

1. Opened a command prompt and went to: D:\ProgramData\Safe Software\FMEFlow\repositories
2. entered: git checkout master
got this message:
error: Your local changes to the following files would be overwritten by checkout:
        Scheduled Workspaces/DubbeleCodeMailer/DubbeleCodeMailer.fmw
Please commit your changes or stash them before you switch branches.
Aborting

3. entered: git commit
got this message:
On branch main
Changes not staged for commit:
  (use "git add <file>..." to update what will be committed)
  (use "git restore <file>..." to discard changes in working directory)
        modified:   Scheduled Workspaces/DubbeleCodeMailer/DubbeleCodeMailer.fmw

Untracked files:
  (use "git add <file>..." to include in what will be committed)
        Dashboards/
        Internal-4f281d882ec3/
        Samples/
        Scheduled Workspaces/TerugmeldingenEmailer/
        Utilities/

no changes added to commit (use "git add" and/or "git commit -a")

4. entered: git stash
got this message:
warning: in the working copy of 'Scheduled Workspaces/DubbeleCodeMailer/DubbeleCodeMailer.fmw', LF will be replaced by CRLF the next time Git touches it
Saved working directory and index state WIP on main: 234a57f Nieuwe versie met Sharepoint transformer

5. entered: git checkout master
got this message:
Switched to branch 'master'

Now it all works fine. 
I probably lost some commits, but I'm happy it works again.