I get an error and cant join any servers

[pepsii 0]

[TheCoon 21]

I've had the same issue as you. The reason i was getting this error was because I changed my mod path (I moved my mods onto my hard drive) Therefore in order to fix this issue you need to direct the launcher to the correct files. 

For example: http://prntscr.com/a6rqyt

AS shown above you can manually set the path, this should fix this. Let me know if it works / doesn't work