Hi Rabanus,
the ST05 trace would have been an option and certainly the work process trace (e.g. accessible via TX ALXX in DIR_HOME = /usr/sap/<SID>/DVEBMGS00/work). There should even be a way in the DBACOCKPIT via the indexserver trace of HANA.
However, as I can see from you post below, you managed w/o my further help 
. So see my post as "additional info" for others who might stumble of the similar issues.
Cheers,
Jasmin