I have it sometimes, that all of a sudden my input lags.
Symptoms:
Your character doesn't stop running,when you release the run key.
You hit the jump key and it takes ~3-5 seconds before the game responds.
Direction change with the mouse lags.
The framerate doesn't drop a bit, the game doesn't studder, the sound works flawless

If I suffer through this for ~2min I can play without problems again. In a gaming session of maybe 3 hours, this might happen once.

I never had this problem in another game. It is mouse AND keyboard input that lags.

I thought it might be because the mouse USB (G700) is connected to the USB hub of the keyboard (G15). But this never happened in any other game.

Does anyone have the same problems or is there anything I should change in general? I thought about getting a USB extension lead and connect the mouse directly to the PC then :/

I never really play the mouse wireless, but the dongle is always connected to the back of the PC.

Not that I can recall being reported. If you have an old mouse handy I'd try it, but I'm not sure what else to suggest...

Try emailing supportdks@larian.com, with a description of the problem and the report.xml file generated by the Divinity 2 support tool.

This is just a thought and it may not apply, but I use a wireless battery operated mouse and often, just before the red light appears to tell me my batteries need replacing, I get this type of problem.

If your mouse has batteries, it may simply be they need replacing, or recharging.

I never really used the mouse wireless. If the mouse is connected to USB, it will use this for data transfer and not go wireless (I tested this actual by not plugging in the dongle part but just USB).
However, last weekend I did not connect the USB (plugged into the keyboard hub) and used it wireless. The lag did happen here as well.

If it would be just the mouse, but the keyboard inputs lag as well.

keyboard-> usb, mouse->usb, lag
keyboard-> usb, mouse->wireless, lag

I'll grab some keyboard/mice from work to test. I'll do just that... sorry that I am not so quick with the troubleshooting.