Sometime today, something changed and now instead of adding spells to my spellbook, I can only "Prepare" them from that menu. This causes other errors and warnings. It happens both from the character sheet and from the character builder tools. I hope this bug can be fixed as quickly as it was introduced! Things were fine last night, and today they are not.

For Wizards — and feel free to correct me here, but I'm assuming you're playing a Wizard because that's the only class that uses a spellbook — you would add spells to your spellbook from the "Add Spells" section and prepare them from the "Spellbook" section. Are you saying that "Add Spells" section isn't there anymore?

It's fixed now. Under "Manage Spells" > "Add Spells" the list of spells had a button next to them with the options "Prepare" and "Delete" instead of "Learn" and "Delete". Now it's fixed though and we can add spells again properly.

I have had a player in my campaign happen to him today.  When he goes to learn new spells after having leveled up, he's only given the option to "prepare".  No option for "Learn" is present.

That's me! I'm that player. 

Did you do anything yourself to fix it, or did it just resolve on its own?

I think it might just be a text swap. I was able to check my spellbook after "preparing" the new spells and they were there. I think it is just putting the word "prepare" instead of the word "learn." BUT! It is also preparing the spell, taking up one of my prepared slots.