Check the version build of your FIM box. If it is older than (4.1.3441.0) then there is a hotfix from Microsoft which is improving FIM from this kind of error you should install.
A hotfix rollup package (build 4.1.3508.0) is available for Forefront Identity Manager 2010 R2
https://support.microsoft.com/en-us/topic/a-hotfix-rollup-package-build-4-1-3508-0-is-available-for-forefront-identity-manager-2010-r2-af96b001-c442-cfd1-5b78-180278ebc7b6
Check Event Viewer on the FIM Sync machine to check for any clues that you might help us in converging on the troubleshooting.
Hope this resolves your Query!!
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