This is a very common error when it comes to mods or failed update installations. It means that the server you are trying to connect to is running a different game version than you are.
Not all mods will change game version, but the ones with a altered Cshell.dll located inside the mod's rezfile will.
Examples here would be:
• Widescreen patches
• VR patch
• Singleplayer mods that are still loaded
• Anti cheat mods
There are also mods such as 'JAG Sniper Hell' that will not affect version at all.
A good example for this would be the NolfRevial.tk version of NOLF, where they made the game load a widescreen patch.
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