I worked with 14.04 LTS for years. Now I need 18.04 for some serious reasons. I upgraded 14.04 to 16.04 LTS without problems. In 16.04 all worked well. Then I upgraded 16.04 to 18.04 LTS with the same way. I have lost high-resolution GUI. From this step I can only use 640x380 screen. The 14.04 used driver nvidia-304 which 18.04 did not load. I removed this driver. After commands apt-get update and then upgrade and reboot it proposed install needed driver. In selection list there were only two options available - currently used nouveau and nvidia-340. I selected nvidia-340 and installed it. This driver is latest for used video adapter GeForce 8500 GT 512. After reboot I get error message - Plasma cannot use OpenGL. In GUI mode I can use only console. Graphics screen is black. All glx utilities fail with same message:
Please help!:~$ glxgears
X Error of failed request: BadValue (integer parameter out of range for operation)
Major opcode of failed request: 154 (GLX)
Minor opcode of failed request: 3 (X_GLXCreateContext)
Value in failed request: 0x0
Serial number of failed request: 23
Current serial number in output stream: 24
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