Since some OCR-recognition of a pre-exisiting scan by the new ABBYY caused some troubles, I am having about 20 GB more on my harddisk. Where are temp-files stored so that I can erase them manually?
You can look in the most logical location: ~/Library/Caches/DEVONthink Pro/OCR, but files should only be left there if you told it to after it asks you to inspect it in the Finder. Normally the OCR engine itself should always cleanup after itself even in the case of a crash or unclean shutdown (in this case it will do so the next time you run another OCR job).
However, the OS itself may leave many tmp files behind so if you reboot that should be cleaned up.