Can't add blocks after upgrading from 8.6.0 to 8.6.1

I’ve been using Dupal since the 6.x time period so this is a weird one for me.

I can’t add blocks, the little java star thing spins and nothing happens, the window does not close. I’ve tried to add core blocks (body) or custom blocks. Nothing works.

I could add the yesterday. Since I last added blocks I did the following things

I installed Telephone and Actions

I configured the sendinblue module (But it’s been installed for weeks)

I upgraded the core from 8.6.0 to 8.6.1

That is when i tried to add the sendinblue block. No dice.

I’ve since Uninstalled actions, sendinblue and Telephone

Still nothing. No blocks can be added.

Caches flushed, what else can I do?

I made a backup before I upgrade the core, but I’ve never restored from one, and I’d rather fix this.

Any help would be … helpful.

Drupal version: