Thank you. I checked the stack-overflow question and my time-zone and time configuration are correct. Also in this case Firefox isn’t even involved as it’s git (this seemed to be a Firefox specific problem that didn’t occur with Chrome).
Git executed from command line as I’m running Linux.
What got me stumped from the stack-exchange contribution is the ‘change rejected’ bit as I’ve only got a read token. I just didn’t expect that I would need write access for this. Also it may me completely misleading as it was a problem with gitlab.