Well I'll be damned. It's exactly as you said. There were several Queries with some hexadecimal stuff as "ID" instead of a readable Query Name. I did a hard database delete on these and now everything works out as intended.
Do you think it's to be considered a program error that these two select statements differ from each others, i.e. the select statement for selection differs from the one for deletion? I'm asking this because I'm unsure how to formulate the OSS message, since I don't know as of yet how these strange entries came into existance in the first place. I'll keep an eye on it in any case.
Thanks a ton, Mathieu! Many Kudos and ponits to you ;-)
In case you should find out where these entries originate from before me, please let me know!
Cheers, Lukas