If you're getting modified content errors in vanilla as well, the Launcher has not succesfully switched back the shaders file when First Strike shut down. There are two methods that should fix this.
1) Check your \Battlefield 2142\mods\bf2142 folder for two files, shaders_client.zip.bak and shaders_client.zip. Move the .zip file to \Battlefield 2142\mods\Firststrike. Then rename the .zip.bak file to .zip.
2) Open the Launcher, start Firststrike, then shut it down. The files should have reverted back.