An error code can appear for several reasons. It may be a temporary problem, but there may also be a technical problem with the document itself.
Usually an error code is accompanied by text that provides more information (as described in the FAQ above), although this is not always the case.
If you do not find a message with the error code that gives you more explanation, and if an error code still appears repeatedly and after logging out and registering again, it is best to contact us, stating the error code and if possible with a screenshot of the error. We will gladly check this for you.