Hello,

JabRef version: JabRef 3.8; linux 4.4.0-57-generic amd64; Java 1.8.0_111 on Ubuntu 16.04
Steps to reproduce:

1. Create a new, empty database (bibtex mode)
2. Add a keyword under manage content selectors for journal: Test {"U}mlaut
3. Hit Save
4. Check comment in editor: ... Test {\"U}mlaut ...
5. Restart Jabref
6. Hit Save
7. Check comment in editor: ...Test {\\"U}mlaut

So with every save each Backslash is escaped again and again.

1. Save

% Encoding: ISO-8859-1

@Comment{jabref-meta: databaseType:bibtex;}

@Comment{jabref-meta: selector_journal:Test {\\"U}mlaut;}

2. Save

% Encoding: ISO-8859-1

@Comment{jabref-meta: databaseType:bibtex;}

@Comment{jabref-meta: selector_journal:Test {\\\\"U}mlaut;}

I hope this provides all necassary and relevant information. I hope I haven't missed something. :-)