Hi Tim,
thanks for pushing me in the right direction: actally the cfg files were of, yet is seems, the lo4j2.xml got screwed up some how: opening it, chrome returned the same cryptic error that I had received earlier on using the console.
Browing through the contents, I found the very last line to hold a syntax error:
After fixing this, ebverything seems to be working as normal, including fronttail 
I guess I’m gonna post bug on GitHub. This behaviour indicates that there’s either an issue closing references for these config files, or (direct string manipulation) modifications are used without validation / error handling.
Have a good Monday!
Oli