I'm not sure if this is bugged of intended. I wasn't trying to undo copy some files (from the undo context menu). The operation failed because one of the files was open. I closed the file, but when I wanted to try it again, this action disappeared from the undo menu.
Confirmed, that happened for me several times (v11). With luck you can still get affected files out of the general file log, but there are cases you cannot.
If you're lion-hearted, you can try out the LogCommand add-in, it gives detailed logging for nearly everything and allows easier recovery of affected items.
It needs a bit of DO config work until ready to use though. Command: LogCommand (detailed action/items/undo csv-logging)
The Undo log would not really help you retry a failed command anyway. (It's not a redo/retry log; it's for reversing the effects of a command.)
If part of a copy or move fails and you abort the operation rather than fix the problem and retry while still in the operation, you can re-do the operation by selecting the same things and clicking the copy or move button. If it's a copy, you can use Skip Identical to skip the files which have already been copied. (Of course, if it's a move, that won't be needed, since anything that has already been moved won't need moving again.)
Ok, how would one do that? Do you mean the "Reselect Files" menu entry?
Reselect works in some cases, but I was thinking just click on the files like was done for the first attempt.
(Even easier: Don't abort the operation, fix the problem, then click Retry. 
Unless fixing the problem requires a reboot or exiting Opus, of course.)