I am up to two people who got my uploads of their characters' art taken down, as of a day ago. Is this common for uploaders? Or did I fuck up?

Because I feel like I really fucked up. At least the second person didn't chew me out over PM. I respect their preferences and reasons to not have their stuff shared here as well, hence feeling awful. I am at the point right now where I think it's probably better I only share my own commissioned crap. Heavens forbid the artist ask for a takedown of my commissions from them, but even that could happen.

So yeah. Melodrama is melodramatic. Just asking how unreasonable it is and why.

Happens all the time, see takedowns. Primary reason being no permissions asked, because most people don't bother at all.

If you want to continue posting other people's artwork, I would suggest asking for their permissions first before uploading here. Even though it is not mandatory.
This includes asking the artist, commissioner, and character owners (if multiple characters). See topic #25097 on how to draft a permission request letter.
If they don't reply to you, you can send one final followup message before considering that they are not interested.

It's fairly common if you make the mistake of not asking first before posting them somewhere. For shared commissions most people are fine having their own characters show up in the other people's private galleries (FA, IB, Twitter, etc) but are not okay with them showing up in more publicly accessible places like e621.

Personal recommendation: Ask every other character owner and the artist whether they're okay with having their likeness/work on e621 before uploading. It's not a bulletproof plan by any means but the chance for a takedown will be much lower. Either by you already knowing they don't want it on e621 and thus you not uploading it, or that they're aware of it having been uploaded by yourself and are thus fine with it.

Just like with most other areas in life where more than on person is involved: Informed consent is key.