JabRef version 3.3 Linux
Hint: If you use a development version, ensure that you use the latest one.

Steps to reproduce:

1. Try to enter a bibtex key with parenthesis in it - eg: Kaur(2014)
2. Invalid bibtex key is returned

Also this implies that the automatic bibtex key generator with parenthesis no longer works (I have 4000 keys with parenthesis...)

Work around - enter key manually in bibtex tex window.

I checked and ( and ) ARE valid in a bibtex key.

This bug was introduced between 3.2 and 3.3.