Ran into something unusual just now with the FIM R2 Sync install – at the end of the install after having selected a folder to save the *.bin file to, there was a delay of about 30 seconds before I got the following error dialog:
“The Forefront Identity Manager Synchronization Service setup wizard was unable to back up the key set. <hr=0x80131904> … try again?”
I figured there was a permissions problem with the target folder location, so I tried several times, including creating c:\temp, giving full access permissions to everyone, and saving it there – but it didn’t matter how many times I tried I got the same error. Eventually I selected “no” and the installer completed with a success status (nothing in the error log or any warning that the key was yet to be saved).
At this point I simply ran the Synchronization Service Key Management program and successfully saved my miiskeys-1.bin file without a problem.
So I still have no idea what the error actually was, but if anyone does have the same experience, rest assured that you can still happily skip this bit so long as you remember to do it straight afterwards using the utility.
IdM for Real 
I have just had a similar issue installing the FIM Sync Service 2010 R2. installation has completed but when I try to backup my keys it errors saying:
“The operation encountered an error and cannot be completed.
Error: the user name or password is incorrect
Error Code: 52e ”
“The Forefront Identity Manager Synchronization Service Setup Wizard was unable to backup the key set. The username or password is incorrect. Do you want to try again? ”
However I was also able to export my keys using the Sync Service Key manager tool.
Same here, in the far flung future of 2018 running MIM 2016 SP1 :-\
@Bob, I encountered the same error recently. Solution: Uninstall, give MIMSync service account local administrator rights to both boxes (the box you install MIM sync service and the database), the installation completed successful with key set saved.
it work for me thanks..