@kolaente Sorry, I should have been clearer that setting the time zone was what got the app running again. I don't recall clearing any cache, despite @girish's kind advice. I'll try disabling the cache, even though I can't say I've noticed any slowdowns. It either has worked well, or not worked at all! Thanks.
For future readers, adding a link to the upstream discussion here - https://community.vikunja.io/t/long-time-regular-main-user-being-asked-to-confirm-email-address/195/
@humptydumpty @scooke I have provided the upstream author backtrace at https://community.vikunja.io/t/400-bad-request-on-first-namespace-creation-error-500-on-api-v1-info/170/15