Dear Sander
Similar type of problem i have faced. In my observation i found when the user making the entries very fast it is getting hanged and when the user doing same entries in a slow mode it is not getting hanged.
I have checked the temp files, when we are printing or creating the document temp files are getting created and in few seconds it is getting deleted. before automatic deletion of the temp when you are trying to add other document then it is getting hanged.
Hanging will happen if there is no time difference in between the entries.
This is my observation only from my client users.