Try a fresh install in a new directory (e.g. Rockstar Games2 instead). San Andreas can be a bit funny like that.
I really do not believe it has something to do with the game itself. Putting 2 and 2 together, I haven't installed any new mods before the messages started showing up, I've used pretty much the same mods for months now, there wasn't a problem. I do not receive any errors or whatsoever when playing Single Player so it seems to be a client issue.
Like I said, I have tried a new re-install, installed the game in a Rockstar Games > SA > GTA San Andreas (Another directory) and still had this on a clean version. Issue must be with the SA:MP Client, and as TriHard89 advised the problem seems to be with custom signs, I get the errors around Pershing Square and the custom signed areas.
If anyone has a link where I can download these basic fonts and "re-install" them it might probably help fixing the issue, unfortunately I was unable to find a pack of basic fonts, just custom ones.
PS: This started happening shortly after I updated my NVIDIA drivers, so if anyone who is using the most up to date ones and not having the issue hit me up.