What does a section perpendicular to the bend look like? Is it a closed section? If not, then it's a geometry issue requiring further investigation. The unbend fail hints at a geometry issue, but check it nevertheless.
You said it is a copy of an original part and then mirrored? You could be running into a accuracy issue related to the bend table. Is the part file you copied things to the same?I.E. units, accuracy? Is the bounding box of the failingpart the same as the first?This will impact the success rateof wacky features like that.
Other than that, it looks like a simple part. Maybe model the oppostite part from scratch and see if get the same failure?
If you are trying to make an identical mirror, I strongly suggest you use the File>Mirror Part command. Make the first part exactly how you like it for that hand, then mirror the entire thing. Besides the benefits of avoiding issues such as these, if you need to make a revision you only need to modify one model tree and not two.
Without seeing the feature itself it is hard to say what is happening. I have had some very bizarre graphic issues when trying to modify the corner of a rounded deformed feature.
As for the accuracy, the absolute or relative accuracy has not given me troubles with features like this. When I do have a problem, the feature fails "for no reason." I change the accuracy to the smallest allowable absolute accuracy and have always been okay.