sts4: Getting Could not instrument class xxxxxx_$$_javassist_0: Class versions V1_5 or less must use F_NEW frames.. after upgrading from 4.5.0 to 4.6.1

Even though the project launches successfully, I’m getting a bunch of these warnings for my model classes.

Could not instrument class xxxx_$$_javassist_334: Class versions V1_5 or less must use F_NEW frames..

All I did was to upgrade STS4 from 4.5.0 to 4.6.1. So far, I’ve been reading about javaassist, m2e and other possible solutions but I really haven’t made any progress. Please could someone explain to me, what exactly that error/warning means and how to avoid it, apart from switching back to the older version of STS. If you need more info about the installation please ask me. Thank you!