I have no idea Judy. The antispam addon triggered it. I installed it to avoid spam from the contact form but it didn’t work and I’ve had to disable the contact form altogether.
I must say I’m very annoyed with the quality of addons and management when it comes to development of phpbb. At least half of all addons I’ve tested don’t work - maybe for older versions but that should be notified on the website. This antispam addon apparently works, but not for the correct problem.
I found a setting now so that it can’t happen again for members who have previously posted.