Well, I don't know whether the 'list of forbidden system ids' does vary for various SAP products and versions.
As is don't know yours, I cannot say for sure whether my idea does apply to your problem.
But here is one list I found:
On page 29 it says:
Make sure that your SAP system ID:
....
Does not include any of the following, which are reserved IDs:
ADD ALL AND ANY ASC AUX COM CON DBA END EPS FOR
GID IBM INT KEY LOG LPT MON NIX NOT NUL OFF OMS
PRN RAW ROW SAP SET SGA SHG SID SQL SYS TMP UID
USR VAR
regards