For some reason when I change the name of the deck, it shows the saving as a success, but then it goes back to its original name, then after I change the name and save and try to add a card, I get this odd error. I have not been able to reproduce this in any other instance.
The video is in the the link here, it is unlisted.
This is caused due to the name being larger than the maximum allowed character limit, a limitator was added to the deck edit input field and an check for it afterwards and the procedure to save the deck changed a bit, fixed.
You have a limit of 20 characters to a deck name. There’s no need for longer than this since you have tags now. You can have as many tags as you want for a deck and use them to filter and find decks. Games like MTG Arena also have 20-25 chars for deck names.