Try to define the same material in Job Preferences under a new name. Restart Robot and open the file again. Can you assign this new material?
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To make sure I understand the situation correctly:
1. You can change the material of a particular bar to any but one from the list
2. If you define a new material in Job Preferences (give it a different name then the existing 'corrupted' one), save the file, restart Robot and open the saved file you are not able to assign this new material either
Is this correct?
Make a copy of a file and try to open it in the repair mode. Is it the same? If yes, isolate this bar and save it in a separate rtd file. I'll try to check if I'm able to assign a newly created material to it.