Suddenly lots of bugs in my WP installation?

Hello,

I’ve been using WordPress for over 4 years without any issues.
But recently I get error messages from various sources (mostly plugins).
As all those plugins used to work just fine until a few days ago, I suppose it’s not a problem within each plugin, but something happened to my WP core installation.

Just some examples of what I’ve noticed thus far:

– Jetpack:
cannot connect to WP anymore (32700 error)

– WordPress Editorial Calendar
Error msg: “An error occurred while loading the calendar:

The calendar was not able to parse the data your blog returned about the posts. This error is most likely caused by a conflict with another plugin on your blog. The actual parse error was:

JSON.parseIt”

– Appearance – Themes:
When I try to “add new” themes I get the following error message:
“An unexpected error occurred. Something may be wrong with WordPress.org or this server’s configuration. If you continue to have problems, please try the support forums.”

Clearly there’s something wrong with my WP installation.
I admit that I modified quite a few files in the past few days (e.g. htaccess, wp-config, funcion.php and some of my javascript files (my own, not the plugin ones).
I tried to undo my changes, but then my site broke, so it’s impossible for me to figure out what’s wrong.

PLEASE, can anybody help me figure out what to do?
There’s possibly a lot more buggy things going on.
I NEED to fix this sooner or later.

Thank you SO MUCH! 🙂