I found an easy solution for LinuxMint, which should also work in Ubuntu and ubuntubased Distros.
Go to your terminal and type in this command: lsmod | grep rt
Your output will lokk something similar to this one:
rt2860sta 488820 0
rt2800usb 37372 0
This gives the hint to the solution:
Steps to the solution:
open your Terminal and first type in this: sudo modprobe rf rt2800pci
Then afterwards type in this: sudo modprobe -rf rt2860sta
As the next step follows this command in your terminal: sudo modprobe rt2860sta
Just as you type in this, your networkmanager connects automaticly and comes back to work!!!
But to close the whole thing up, you need tot ype in this command, for finishing up:
echo blacklist rt2800pci | sudo tee -a /etc/modprobe.d/blacklist.conf
Then you are d one and your networkmanager will work for you again.