Could not find or load main class after update of jdbc driver

As a developer (mainly C++) i inherited a Java project, which until now build fine.
The only thing i changed is the jdbc driver mssql-jdbc from version 8.4.1.jre8 to 12.8.1.jre8.
Now i have a “could not find or load main class” error when starting the application and i am lost.
The pure eclipse build without ProGuard runs without error.

Has anybody seen this particular effect or can point me in the direction where to look?

I did the same thing with jdbc drivers for oracle, postgresql, mysql, hyper, h2, no problems.
As far as i can tell the all the neccessary -keep options are set properly.
Maybe the used plugins need updates?
As i am not familiar with the inner workings of maven i did not want to update
all plugins at once.

Some info about our build environment (as my C++ brain understands it):
Java8 (openjdk), we build via maven, use eclipse for editing.
The application is put into a jar file, obfuscated and packed by proguard-maven-plugin
2.6.0 - ProGuard 7.2.0 and then put into a Windows exe via launch4j 2.1.2.