Hope this can help anyone else with the same problem...
When you create the docker you set the location of your files in "User Files"
Note that the container is going to access this path through /var/www/html/data .
When you are setupping the account you are now asked where to add the Data folder:
If you change this path you will not point to the data folder you defined earlier, this will cause the interface to issue the message you are seeing.
PS: Other problems may cause the same, but I dare to say this is a common mistake.