Doing as most IT helpdesks do these days, I had a quick online search, and on PHPBB's own forum, saw this thread which touched on the invalid form error, and a possible cause of differing.
https
://www.phpbb.com/community/viewtopic.php?t=2554381
In particular, the post by EA117 » Sun May 31, 2020 3
:22 pm, talks about how the same IP address can be mistakenly assumed to be different. I don't understand the IPv4 vs. IPv6 discussion and how to investigate it for yourself, but there are two actions it does suggest, that are easy to do, and if nothing else, help eliminate possibilities. I've added a 3rd:
- Ensure the remember me function I mentioned is running ok (it looks like it's activated for all on this site, but checking what it's showing can give insight)
- delete cookies. After 'switch it off and back on again', this is a cliched IT support 'fix', but it can be a legitimate fix, and is easy to do.
- Try a different browser, so see if it's specific to the browser you use.