Web Publisher "Couldn't open database"

The past days I receive the following message from running Web Publisher Web-User-Interface: “Couldn’t Open Database”. I already started to look for any solutions in the internet, but it seems noone has this issue before.

Does anybody of you know what the reasaon for this error message is and how it can be resolved?

Within Reference Manager the sevice could be stopped and restarted and the web page of Web Publisher appears, but it shows beside header and footer of web publisher web page only the above mentioned error message

Thanks a lot in advance for your help.