Achievements seem to lock up Forge

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
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