>> Also, check if the Firebird database files (including the security
>> database) are excluded from your virus scanner.
>>
> That's quite an idea. We'll try as soon as possible.

that suggestion helped a lot. We found, that the main-databases were already excluded from virus-scanning, but the security-database was not. After adding a further exclusion for the security-database we saw a sharp drop in "create file failed" errors. They have not gone completely (at least on HDD-based systems) but have dropped so far, that we are confident, it actually is a load/setup issue we can handle with further optimizations.

Thanks a lot for your help,
Jonathan