You are probably cutting complex geometry, and the cut line has landed in an unhappy spot.
This can occasionally occur. you need to move the cut line slightly, or modify the portion of the geometry you are cutting.
The trick is finding the area that is causing the problem.
To do this, cut your part (temporarally) in half (keeping both sides), cut it in the oposing direction to the cut that is having the issue.
Then try to do your original cut on each half, if they both fail then you got problems.
If only one fails, then you have narrowed the offending area down by 50 percent.
cut the failed portion in half again (also perpendicular to the original desired cut)
now try your original cut on the remaining two quarters, if only one fails, then you have further narrowed the
problemed geometry. by now you should be able to spot the problem, usually an area where a lot of faces come together, or strange fillets/patches.
usually you don't know exactly how to fix it, just make a few changes or simplify the suspicious area and try to cut again untill it works. this method has corrercted failed cuts for me many times past.
Don't save the part. Reload the original whole part and fix the area if you have located it by now.
now you can do the failed cut without errors. hopefully.
T.S.
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