Quartus internal error (an old Quartus version)

Dear all,

I have been using an old Quartus version like 8.0 for some reason.

When I modify and compile a design, I am seeing the following error very often when Quartus is generating assembler (generating .sof file).

The modification was simple like renaming bus using wire component (I am using schematic environment.)

Does anyone have any experience or reason for this?

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Internal Error: Sub-system: CUT, File: /quartus/db/cut/cut_lcell_util.cpp, Line: 6687

Error in merging. Locked bit differs

Stack Trace:

0x2AE31A : CUT_LCELL_UTIL::debug_output + 0x174A (db_cut)

End-trace

Quartus II Version 8.0 Build 215 05/29/2008 SJ Full Version

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I am attaching the screen capture as well.

Thanks.