I just tried it in my SW18 SP05 and I can still use CTRL+C / CTRL+V to copy lines out of a DWG into a sketch in solidworks.
Why you can't at this moment is difficult to say.
If rebooting your system doesn't help, check if your settings is the culprit. In other words either import someone else's settings or revert back to the default settings. Alternatively, open up Solidworks RX, first tab Home, bottom of that page you'll have the option to launch it while bypassing tools/options settings. If you still can't get it to work then you'll best bet is contacting your VAR.
Have a nice one
Thanks for your advices!
Unfortunately any of these didn't work, but I appreciate your help.
You can also save the sketch as a .dwg and then open that file in SolidWorks. It will ask you how you would like to open. Select "Import to a new part as:" and select "2D sketch".
It works, but need to save autocad file as .dxf not .dwg.
Anyways thank you!
Have a look at where the entities you are copying are located, they might be at a tremendous distance which is what stops SolidWorks from being able to bring them in. Something like 1.6666E+06, 2,3333E+06
I always moved whatever I had to copy/paste to 0,0 to make sure that it would insert itself at my origin and to also not get the bug that the sketch can't be inserted.