Configuring JabRef on different computers for using a shared database including pdf files

Since I host my literature data base and the associated pdf files on a shared directory, I wanted to make JabRef use the same database and link the pdf files correctly on different machines (MacOS, Linux, Windows). Took me a litte bit of trial-and-error, but here's the recipe:

1. put your JabRef bibtex file and the associated pdf files on a shared drive (e.g. a samba share on a file server, a Dropbox folder or whatever you like)
2. make sure, that all directory entries in the dialogue "File --> Database properties" are empty
3. on each machine, open the database via JabRef and set the pdf path in "Options --> Preferences" in the section "External programs" in the field "Main File Directory". Depending on your configuration, you might want to fine-tune directories with the other free fields in that dialogue. 

Tested with Windows, MacOS and Ubuntu.