@mark & @ralfhiemisch
Maybe there is something similar in your two systems or setups that is somehow contributing to the problem.
Maybe if you could both post your computer specifications (Mac vs PC, operating system, CPU, RAM) and a few other details, there may be some pattern or link between the two.
If there is anything non-standard about your Lightroom installation (such as a custom install location), please let me know.
Also, please let me know where the images you are using in Lightroom are located when you select the Juicebox web engine (for example, on a local hard drive, a NAS drive, a folder synced to an online file sharing service or somewhere else).
The more information we have, the more likely we are to find the cause of the problem.
When the error occurs, you could try pressing Ctrl + R (or going to 'Web -> Reload' from the drop-down menu at the top) to reload the live preview to see if this helps.
Lightroom serves the live preview from a temporary folder. On a Windows PC, the files for the live preview should be found in the following location.
C:\Users\Username\AppData\Local\Temp\AgWPGPreview\
Whilst Lightroom is open and the error message is being displayed, go to that location (if you have a Windows PC) and check that there is a file named 'config.xml'. You should be able to open the file in a text editor to check its contents. Please let me know what you find.