Canon T3i flash doesn't work as it shows "busy" before a previous "failure" need help

Hi, I recently bought an used Canon T3i, really good physical condition overall, and with only 15k shots. But the flash had a previous issue where it didn't let me deploy it because it seemed like the camera detected a mounted flash on it, obviously checked and so, and after a lot of things and workarounds I could still use an external flash, but wasn't able to use the integrated one.

Now, yesterday, messing around with auto mode the flash deployed automatically, but doesn't shoot, as whenever I hit the button it shows a Busy flash message. I tried with 2 different batteries, and obviously isn't the sd card. But I can't seem to manage to get it working.

I know many would tell to send it to tech revision, but you'll understand that i'm trying to fix it by myself first, before even paying for a fix.

If anyone knows something, I would be very grateful.