Thanks for answering. Just tried your suggestion with different browsers
(firefox, chrome, brave) but the error message still appears when adding a
new user.

By the way, i justed noticed that if you click "back" followed by "cancel" you
enter the logbook as the user you just added. It seems that although elogd
throws an error, the user is added. Weird.

Since most of our users don't trust open source software, the error message
will only confuse them. So, is there any way to avoid this message from
appearing?

Perhaps a few words about our use case:
We are a german non profit organisation in disaster control. Since 2007 we use
elog as mobile mission journal in our unit. We adapted elog to run from
usb-stick, so all our first-responders have to do is to plug it into a laptop
and they are ready to go.
We also hand the adapted installation to other organisations like local fire
department or red cross units and offer them a short (2 hour) course on how to
use it. Since the windows-release is the most requested, it would be confusing
for them if an error message appears every time a new user is added.

So if you have any further ideas, I would appreciate that.

Uwe