I have just set up a moodle for my school for trial run with a few subjects. There have not been many troubles. However, today, when I tried to upload some files in a course as resources, a window popped up and show the following error message:

Error establishing a database connection

It is strange as it works well with all other activities and resources. I found that all functions involve uploading return the same error message. However, that doesn't include uploading user lists, user picture or activities like "upload a single file". The error only appears when it involves the pop-up window that shows the directories and the files in the resource database (I suspect).

Anyone has got an idea what went wrong?