Quartus Internal Error Advice

[Altera_Forum]

When compiling I get the following error in a window. It happens during the Fitting section of compile. 

 Internal Error: Sub-system: CDB_ATOM, File: /quartus/db/cdb_atom/cdb_atom_sys.cpp, Line: 1338 implements(type) Processors in use: 4 Stack Trace:  0x6a963: SIN_VALUE_UTILS::get_io_pad + 0x1f3 (TSM_SIN)  0x606a9: Sin_Unload + 0x8bb9 (TSM_SIN)  End-trace  Quartus II Version 9.1 Build 304 01/25/2010 SJ Full Version Service Pack Installed: 1  I've been getting different errors recently, but this is the first of its kind. I'm trying to set up a pseudo-differential signal on a Stratix III.  Any advice as to where I can find info on this error? I've been looking, but haven't found anything.

[Altera_Forum]

unfortunately, most internal errors are hard to debug without Altera's help. i recommend creating a small project that exhibits this problem and attaching it to a Service Request 

 note that 9.1 is getting to be a bit old, support may be limited

[Altera_Forum]

Most internal errors can be removed by deleting "db and incremental db",try to delete and see if it works.

[Altera_Forum]

Thanks Jai Shree!! I usually restart Quartus and it works!!