Everytime I alt+tab ac.

[Cemer]

Reinstall the game Huntsman to see if it makes a difference, when you reinstall make sure to get the newest binaries from the site

No. I've found away to get around it.

So, somewhere SOISOISOI MCS stated that this happens when using too much memory. So, every time I want to do coop, I restart my client. Haven't gotten this error in awhile.

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EDIT: Wait never mind. Alt+Tab doesn't use memory (I think). Do what pwnage said. Sorry sir.