Hello Lars,
the CheckStyle folks have (accidentially) broken the binary compatibility of their extension API between versions <=5.8, 5.9 and >=6.0; see https://github.com/checkstyle/checkstyle/issues/505 .
You should be extremely careful when recommending CS extensions (like SevNTU) and document very clearly which version (and build!) of the CS extension works with which version of CS + ECLIPSE-CS.
Alternatively, to avoid sh**loads of user problems, you could discourage the use of CS >=5.9 and revert to CS version 5.8 until the "dark age" of CS is over. Alas, you will lose the Java 8 support that came with CS 5.9 :-( .
eclipse-cs 5.8.0 is still out there for download @Sourceforge if someone needs it. eclipse-cs 6.2.0 will come out soon with the updated Checkstyle 6.2 core which allegedly fixes those issues.