Started re-playing GOTY edition, and there are lots of red diamond exclamation points. What is going on? How do I fix this? I have googled it, and says it's a mod issue. Is this wide spread? Also said it was a texture issue.

Yes, generally it's a mod issue that this usually occurs with. Though it's not exclusive to mods.

Basically, it means the game can't find the files -scifically meshes- that it needs. Hence the diamond. Texture issue would usually be everything either one block colour (Often purple, black or white)

Try reinstalling the game or installing if you haven't already.

What system are you using?