I have tried to edit my players' character sheets this afternoon to add feats that I have access to but they do not. I have purchased the books on my account but none of them have. When I edit their characters everything is fine until I add the feat. This happened when I tried to add Resilient (CON), Martial Adept, and when I tried to add Armor of Agathys to a warlock character's sheet for our next campaign. The warlock character is in a separate campaign from the first two. 

I tried editing my own characters and had no issue doing so. This has been a non-issue for the past two years of DMing, so I'm a bit confused why I can't add features to their characters now.

My error code is 4c11ef20b8b043bdb44190e107fca296 or 050c2cab60504fbb97c0f70c2426275f with @dndbeyond/character-app@1.8.4. 

Unless you have Content Sharing for the campaign, you shouldn't be able to add spells and feats to player's characters unless the player owns the source material on DDB. There is a recent issue with DDB that lets you the DM see and attempt to add feats/spells that you own to players' character sheets, causing the error.

If you do have Content Sharing, try turning it off and back on again, then reload the character sheet page, and see if that helps.

Same. This worked for me until recently, and now I can't edit one of my player's PCs. I can still edit other player's PCs, but for some reason, I can no longer edit the rogue.

This is intended site behavior.

By default, a player should only have access to content that they have purchased or is free (Basic Rules).
Content sharing with a master subscription means that the player has access to any content that anyone in the campaign owns, for characters in that campaign only.

Allowing DM edits to select feats for PCs in games without content sharing enabled was a bug, not a feature, as it bypassed the content sharing restriction.

Understood. I've gone ahead and purchased a Master Subscription to share with my players going forward.