Please atatch log.txt file after crash (b4 you restart client).
Or at least paste last 20 lines of it.

Log file

This appears to be a recurring problem for design puzzles with long sequences (say, >200 amino acids) or variable lengths (i.e. those that allow the insertion or removal of segments). My guess is that the former might be a memory/stack issue, while the latter is due to the lack of one-to-one correspondence between the current structure and the solution being loaded as a guide.

Yes, looks like it happen when guide and current puzzle have different length.

On main - it is crashing even if you save the current puzzle and try to load it as a guide (Win7):

Problem Event Name: APPCRASH
Application Name: foldit.exe
Application Version: 0.0.0.0
Application Timestamp: 4babd899
Fault Module Name: game_library.dll
Fault Module Version: 0.0.0.0
Fault Module Timestamp: 4e9f1644
Exception Code: 80000003
Exception Offset: 002037af
OS Version: 6.1.7601.2.1.0.768.3
Locale ID: 3081
Additional Information 1: 0a9e
Additional Information 2: 0a9e372d3b4ad19135b953a78882e789
Additional Information 3: 0a9e
Additional Information 4: 0a9e372d3b4ad19135b953a78882e789

Looks like fixed, no more reports.
Closing.