LESS Builder: neverending Working

I am using TTW Diazo Theming. When I change anything in .less and click on "Build CSS", LESS Builder starts to work but it never finish, "Working..." is always there. This behavior is the same on any .less file.
Could you please advise me what can be wrong?



in short: was (almost) fixed in 5.1.1 (although if you rely a lot on Tinymce you may want to use workarounds to 5.1.0 outlined in the Github issue and stick to 5.1.0 since 5.1.1/5.1.2 suffer of a Tinymce update with several side effects)