I regularly have to login to WL to further use it. For some time now I have the problem that all my settings are lost every time I have to do that. What could be the reason?

Let's agree on definitions first. What are the "settings":

1) your saved credentials in Wealth-Lab Developer's Auth Wizard dialog?
2) all the Strategies, data and program's preferences?

All preferences (F12) are set to default and everything under position sizing.

In your case, apparently the WealthLabConfig.txt file is read only (see here) or more likely corrupt. With WLD closed, get rid of the file at:

C:\Users\Windows User Name\AppData\Roaming\Fidelity Investments\WealthLabDev\1.0.0.0\Data\

You might have to enable hidden file visibility to find the file, if not done already: How to show hidden files in Windows.

P.S.

If it were #1, the answer would be to create a support ticket for resolution.

For #2, and also if the Activation Wizard dialog pops up each time WLD is started, it'd be the forced clearing of the system AppData\Local folder after exiting the program - which was found in some (paranoiac) security configurations (in corporate environments).

Matthias, have you had success in fixing it?

Yes thank you

Good to know it's helped. You're welcome.