[UPDATE] This is solved! You just need to update your synapse.conf and nginx conf for your synapse domain. See here for more details. It may also be fixed in a package update which seems to be being worked on.
I seem to be having this error as well. As soon as I edit the home server in the Element Android app or Element Web, the only option it gives me is “Sign in with CAS” which takes me to the Yunohost SSO login and then never returns to the app after signing in. It seems that the URL it’s opening is just DOMAINNAME.TLD/yunohost/sso/ without any kind of asking for a token to return to the app or anything.
I’ve updated the issue you listed with logs and further details. @ericg I see you seem active in synapse_ynh. If there’s further info/testing I can help with, just let me know.
@chateau Could you maybe update the title of this post to reflect that the issue isn’t just resource usage, but now not being able to log in?
EDIT: There seems to be more discussion of the “not able to log in” issue over here.