JabRef 5.1--2020-03-23--67daf36
Windows 10 10.0 amd64
Java 14

Steps to reproduce the behavior:
BibTeX-Key conaining dashes are reported as illegal BibTeX-Key

But according to @Siedlerchr in #4709 (comment)

Well, dash is a perfectly legal char in a bibtex/biblatex key: Forbidden are only these symbols: " # % ' ( ) , = { }