I can't add movies to existing collections (empty list)

[Kiwa]

Hmmmm.  Nothing out of the ordinary.  Can you open a browser console (F12), click on the "Console" tab, and replicate the problem?  That should show you any issues when it populates the list of collections to add to.

Also, do you have any custom CSS or themes applied?

Nope, my custom CSS box is empty. Also default theme.

Could be any addon?

[TheDreadPirate]

I have an idea, but I need to know something before I start going down this rabbit hole.

How many collections do you have?

[Kiwa]

I have an idea, but I need to know something before I start going down this rabbit hole.

How many collections do you have?

Hahaha. I only have one collection, the one I showed you on the first page. "Barbie Collection"

[TheDreadPirate]

I'll have to do some testing when I get home tonight. I'll get back to you tonight or tomorrow (US EST).

[Kiwa]

I'll have to do some testing when I get home tonight.  I'll get back to you tonight or tomorrow (US EST).

No problem. If you need access to my Jellyfin I can create an account for you, but I will need to contact you privately.
I'm from Spain so UTC+1
Thank you for taking the trouble.