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Was playing a game of Commander in which I activated several Planeswalker's Ultimate abilities in a single game (Thank you, Doubling Season) but after the game, it showed one achievement, followed by a complete seizure of the program, requiring me to shut it down through Task Manager. I don't know what happened, or if this is a known issue.

If I earn just one achievement, it doesn't lock up the game. But if I earn multiple, it seems to do so.

At the moment, this is the best info I can give, as I don't know any of the other tech necessities.

Forge Version: 1.5.55-r-1
Operating System: Windows XP 5.1 x86
Java Version: 1.8.0_66 Oracle Corporation

Any specific Planeswalkers?

The four I used in that match were Narset Transcendent, Chandra Nalaar, Jace, Architect of Thought and Ajani Goldmane.

I had the same thing happen.

Played quest mode, got my 10th win in a row.
Achievement showed up, I clicked ok. Game froze. Music is still playing, but the game window stopped refreshing and as soon as I switched to another task and back, the window vanished for good. I can still make the game active but it's not showing anything on screen anymore.

I don't know if any other achievements were unlocked, it froze after the first one.
Had to close from task manager.

...most likely unrelated, when I try to run forge.exe it says I don't have Java available even though I do and running the .jar file works normally.

...might be related but probably also isn't : pretty often when I try to click stuff in card lists especially the card shop, there is no response until I switch task away and back to the game. Almost as if the game lost access to the mouse despite still being active.