Hi Joachim,
Thank you for posting your query. According to your description, here are some suggestions and more details would be grateful.
Are you using Windows defender? If so, you could try to disabled with a GPO.
What is the version of your server and the fslogix you are using? You could try the latest version of fslogix. https://video2.skills-academy.com/en-us/fslogix/install-ht#download-fslogix
Additonally, you could refer to Trouble Shooting Guide for FSLogix products for general troubleshoot. https://video2.skills-academy.com/en-us/fslogix/fslogix-trouble-shooting-ht
If you need further analysis, you could contact Fslogix support team for more help.https://social.msdn.microsoft.com/Forums/en-US/14ea5f6f-760b-4ad2-83cf-23c3cbc1bcc4/how-to-open-a-fslogix-support-request?forum=FSLogix
Best Regards,
Grace