Hi Urs,
this problem looks like an error we just recently found and fixed with versions 7.7.07.48 and 7.8.02.39 (already available via SCN afaik). The fix in 7.6 is not yet delivered to anyone. So if ever possible upgrade to one of those versions to make sure to not hit this problem again and again. These versions also contain 2 more corrections in index implementation which might lead to index corruption, too.
What is the problem in your case: counting the leafs for a join select did not consider empty pages at the right edge of a tree. This could lead to a crash. But normally such a system would come up again after the crash. Therefore I would be interested in the log files of the damaged system (everything in the wrk-directory of the database like knldiag, knldiag.err or KnlMsg, KnlMsgArchive, KnlMsg.old - and don't worry about the formatting of the files, I will be able to format and read them here in the labs). To upload zip the files and use the following container:
Sorry for all the inconveniences.
Best regards,
Steffen