This is a known Microsoft's bug in SQL 2005. This has been addressed in Hotfix for SP2 Cumulative Update (to be applied to SQL 2005 SP2 Enterprise Edition)
Here is the hotfix relating to it (and some other issues in SQL 2005) : http://support.microsoft.com/kb/933265
The "Severity 20 error" should generate a dump file on the SQL Server in the same folder where the SQL Server logs are kept. E.g. ...\MSSQL\LOG in addition to the .mdmp there is a .txt that usually has enough clues to help solving the issue.
This issue requires the hotfix to be applied.
(from Microsoft): "To resolve this problem, obtain cumulative update package 2 for SQL Server 2005 Service Pack 2 (build 3175). (...)
(MS KB) 936305-Cumulative update package 2 for SQL Server 2005 Service Pack 2 is available"