Apple - Error code -65541

I found this to be an error code generated from Synergy. I was having the same exact issue. Same error with a dialogue box that gave me no indication where it was coming from. After a little bit of time on Google I found a few bug reports submitted concerning the error and Synergy. Once I tried to interact with Synergy I found that it was basically frozen. So I fired up my Activity Monitor and force quit the application. Once I did that everything was fine. The Error finally went away.

I haven't looked into the Synergy logs or anything, so I can't exactly tell you what in Synergy caused the error, I just know Synergy is the culprit.

Just for reference, I'm using a MacBook Air running 10.10.3