Excuse my bad english...
Selection of the adress file in the serial letter assistant didn't work. You could not press the OK button in the file selection for the adresses after selecting a .ods file.
The problem is, that writer tried to create a .odb file in the working directory. Since our users have a (beginning) working directory in which they have no write-permissions, the creation fails. There is no error message, the OK button is left gray and nothing happens.
At least, the assistant should give an error message. But more we encourage to use the temp directory for the .odb files instat of the working directory. The .odb files are not deleted in the working directory and only used for the temporary purpose of creating the serial letters.
CONFIRMING - LibreOffice requires write permissions in the current directory in order to create the ODB file, but this is normal beahviour.
As to whether or not it is desirable to delete the ODB files once mailmerge has finished, that is a matter of choice. If they are kept, then they can be re-used for future mailmerges, which I personally find useful.
Obviously, in your scenario, these ODB files are unwanted and would be better placed in a tmp directory where they get removed on shutdown, session logout or reboot.
I propose to mark this as a request for enhancement, and see whether any developer decides to think it worthwhile dealing with it.
I dont think that the ODB files are useful for future reuse. A normal user is not aware of these files, they only see there lists in Calc or any other format. They dont handle with the ODB files. For a user point of view, these files are garbage.