Rules & rule error logging

I configured logging to log my rules in a separate rules.log file, error related to the rules are stull going to openhab.log:
2017-07-07 11:30:06.110 [ERROR] [ntime.internal.engine.ExecuteRuleJob] - Error during the execution of rule omnikdata: String index out of range: -1

I tried adding the following to the config file:
log4j.logger.runtime.internal.engine.ExecuteRuleJob = DEBUG, rules, osgi:*
but that caused the error not to be logged at all.

Any idea what I’m doing wrong here, is it even possible??

The basics: v2.1 op raspbian release

Always when asking questions like this, post the relevant config files.