When are you seeing the error that input is not supported? Can you get a screen shot or is there any other information? If its a black screen that just says input not supported, that is likely coming from your monitor and not the PC itself because the card is outputting in a format the display cannot understand.
Safe mode would not use the same resolution so that may be why its working in safe mode and something in your normal windows configuration is using a resolution outside of what that monitor can support? Then again, I do not think it would work at all if the resolution was the problem...but stranger things have happened.
Also, if its working in safe mode, you at least have a starting point because you can use msconfig (type msconfig in your windows search) and use it to turn off start up items and services. For instance, turn off all the start up items (though nowadays msconfig redirects the start up configurations to task manager) and see if the issue persists after a restart. If it does, then the issue is likely not one of the startup items. Then, try turning off all non-microsoft services. If the issues does not persist with startup items or services turned off, then you know its one of the startup items or services respectively...whichever you had turned off when the issue did not occur. Then go through the items in whichever startup or services list and try them individually to determine which is causing the issue.
