The only thing I tried was to add a new alias to an existing email address (via webfrontend). But I can not exactly recall the very status of the system at that time.
Nevertheless, I made a complete rollback to the old version (by installing the backup). Instead of updating ISPconfig in total, I now only exchanged ClamAV. The above mentioned problem remains for some (quite few, actually) mails. I switched on the logging for procmail, and see that it is working. But I still can see, that mails are handed to procmail. the timestamp of the tmp folder changes, but neither can the mail be found, nor are there any traces in the logfile of procmail.
I know, it sound strange. I'm glad, it happens only for very few mails. Once I find the time, I will write a small wrapper for procmail to see if I can get more details.