The problem obviously lies on the json file. With all of those years, you have never touched nor edited the name of that file, right? then that file is good but the platform doesn't read it correct. If the MEW support still don't reply, post it to the Ethereum forum.
https://forum.ethereum.org/I've never edited those files ever since so I thought everything is alright. The weird thing is that all the other wallets are working fine just this one which contains some decent amount of money lol. i wrote to MEW last night and they are saying that the file might be corrupt and they can't really help with anything more.
OMG, sorry about that.
If that's they've said, we can't do anything about it anymore. As I've said, the problem is with the json file and probably when you have downloaded it long time ago, you have unchecked if the file was all good and not corrupted.
Or, as time passed, it became corrupted suddenly and probably was affected by some apps that you have downloaded.