Hello. Best birthday wishes to your Gran - 87 is well worth a party

I have installed and am running the 1.90 subversion. I replaced a fully operational 1.8.13 (test) environment. All seemed to work well at first, have re-imported a repository from a data dump, can browse to it using URL, can do anything readonly, but when I try and commit anything I get an authentication error. Also when I try and sync with my main production subversion (still at 1.7.

I get this :
svnsync init --allow-non-empty <target repo URL> <source rep URL>
svnsync: E120190: Error running context: An error occurred during authentication
All of this worked fine with Subversion 1.8.13 modules.
I will probably switch back to 1.8.13 for now but I can fire up my 1.90 env if needed for debugging/testing. Any suggestions on how to find out what it is ?
Thanks