Assuming everything was running correctly before adding the service pack you may be able to resolve the issue by editing the asconfig.ini located in the arcserve.6 directory. Find the loader section where the ASPIBD and TAPESVR nlm's are loaded. Add USEABOVE16 switch, save the file and reload ARCserveIT. ASTOP6/ASTART6. OR, if the latest service patch for ARCserveIT has been installed (at time of this writing it is lo70914.zip), AND you are using HAM drivers for the scsi controller(s) with the tape drive(s) attached you can... Switch to CANWPABD. Here's how...

Service pack 3 for ARCserveIT introduced a dedicated CDM. You can switch to this driver 1 of 2 ways.
The 1st method requires the ARCserveIT application to be loaded and accessing the arcserve.exe must be possible. In this case, of course, ARCserveIT will not load, so...
Make a backup copy of the asconfig.ini (by default in SYS:\arcserve.6\) and then
open with NOTEPAD or DOS EDIT and find the loader section. In the loader section you should be able to find a line that looks something like this ->
CSNLM6=SYS:\ARCSERVE.6\NLM\ASPIBD HOST=0 NAME=BOARD0
(note- your CSNLM # , HOST= # , and NAME= may be different and that's ok)
Change this to
CSNLM6=SYS:\ARCSERVE.6\NLM\CANWPABD USEABOVE16 CLEAR
save the changes to the file and close. Edit ASTOP6.ncf and add UNLOAD
CANWPABD either before, or after, the UNLOAD ASPIBD statement. ASTOP6 if
you have not already and then ASTART6.