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Great news that it worked for you.

I hope it stays working. I wonder if you always need to use the standard resolution on an iMac and/or MacBook or it was just a coincidence that triggered something. I fear a little to change it again and leave it for the moment at that resolution.
 
I did just a bit of testing. I can't get it to malfunction by changing the resolution of either computer. It would be great to figure this out. I am not confident it will keep working, even for me.

When I was having trouble, I did find posts discussing Universal Connect only working intermittently.
 
It's not free, but I've had luck with using Synergy (from Symless) instead of Universal Control. UC got temperamental with me and I couldn't always rely on it. TO make matters worse, I then needed to use it on a machine signed into another AppleID (for work) and there's no way to do that.
How are you doing that exactly? According to Synergy website, they don't yet support IOS.
 
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