Here are Formlabs instructions for removing a failed print from an LT tank. Scroll down to Cleaning Resin Tank LT:
https://support.formlabs.com/s/article/Resin-Tank-LT-Maintenance-and-Care
It’s similar to scraping a normal tank (but as you noticed the LT’s are a little more delicate):
https://support.formlabs.com/s/article/Cleaning-after-a-Failed-Print-Form2
Be sure to use a scraper with rounded corners. If you unintentionally gouge, puncture, scratch or indent the tank it may need replacement (see Diagnosing damage and wear on that first link)
If it’s really stubborn, you might try heating the tank in the printer (install it without a build platform and pretend you’re going to do a print, but cancel it before the laser starts) which should make the cured resin a bit more pliable.
I’ve had a similar failure with Rigid and it was a pain to get off, but doable. I managed it without significantly damaging the tank, but there remained a fair bit of ghosting underneath. You may want to position the next attempt on a different portion of the tray.
As for getting better build platform adhesion on your next attempt… Is that a Preform-generated raft, or are you printing directly on the base? If the latter, how large is the raft?
Here are some tips to diagnose adherence issues: https://support.formlabs.com/s/article/Non-Adherence
You might want to filter the resin to ensure there aren’t any failed bits left behind in it.
Contamination in the optical path can exacerbate adhesion issues. Inspect and (if needed) clean the underside of the tank’s clear acrylic window (with Novus #1 plastic polish and a clean microfiber cloth - instructions are in the first link), and inspect and clean (if needed) your Form 2’s glass optical window.
You can also try lowering the Z-Offset a couple ticks:
https://support.formlabs.com/s/article/Fine-Tuning-the-Form-2
How long has the resin been sitting in the tank? If it’s been there for months that might be a contributing factor (again, I’ve successfully done it, but I think they recommend a month or two max). Either way, definitely make sure you mix it well before printing (liberally stir any resin in the tank, and shake the cartridge well). Make sure the resin hasn’t expired.
Here are some more general tips for using Rigid:
https://support.formlabs.com/s/article/Using-Rigid-Resin
Have you had any issues with other resins, or just Rigid?
Also suggest if you’re stuck or concerned, to open a ticket right away with Support as they’d be best equipped to help (and if you damage something under warranty while following their instructions it’s more likely they may be inclined to replace it).
Good luck!