I found this error when doing backup on File System Device. The tape engine crashed and "E3855 Unable to position media" was the message. The problem source is the patch QO64863, which contains a new "tapeeng.dll" file. This dll solves the "E3802-Invalid File Header Signature" bug, but generates a new one.
BAB 11.1 SP1 running on Windows 2000 Server (was able to reproduce on 2003 Server)
BAB 11.1 SP1 running on Windows 2000 Server (was able to reproduce on 2003 Server)