Is it possible that only the problem Mac is running Mojave? If it is then you have to grant permission for the TeamViewer app to control the computer. If TeamViewer is set to run at boot or login, then you will also need to grant permission for TeamViewer_Desktop.
See this article in the Knowledge Base: How to control a Mac running macOS 10.14 Mojave
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Is it possible that the HOST computer is a Mac and has recently upgraded to OS X 10.14 Mojave? The host's user will need to grant permission for TeamViewer to actually control the computer. It does that using the Accessibility functionality of OS X. If the host is set for TeamViewer to run at login or boot, then TeamViewer_Desktop will also need to be granted permission to control the computer. Open System Preferences then go to Security & Privacy->Privacy->Accessibility. In the are to the right under "Allow the apps below to control your computer" locate the TeamViewer row and check the box beside TeamViewer and TeamViewer_Desktop if it's in the list. There is an entry for this in the Knowledge Base with a title referring to "Mojave." Hope this helps...
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I am having this problem as well, but I can do you one better. I have 2 monitors on my mac with different resolutions on each. Not only does the issue you describe occur when I remote from my iPad Pro, even after selecting the proper resolution, shown in settings followed by (Don't Change), it will change the host Mac's resolution every time I swich the control to the other monitor. For those that aren't getting the idea here, I have two high-resolution monitors on the host Mac desktop computer. The remote client is an iPad Pro. This is how the sessions go: 10 Connect: (Host monitors switch from high-res to low-res) 20 Change res back to high-res in TV on iPad, then do stuff using this monitor. 30 Switch control to second monitor (First monitor that was just reset to High-Res "Don't Change" changes to low-res, Second monitor is already low-res from connecting) 40 Change second monitor back to high-res on iPad, then do stuff on this monitor. 50 Switch control back to first monitor (Second monitor that was just set to High-Res "Don't Change" changes back to low-res, First monitor is already low-res despite having been set to High-Res "Don't Change") 60 Change first monitor back to high-res on iPad, then do stuff on this monitor. 70 Switch control back to second monitor (First monitor that was just set to High-Res "Don't Change" changes back to low-res, Second monitor is already low-res despite having been set to High-Res "Don't Change") 80 Goto 20
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