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Haven't had this exact problem before, but Idlemaster is not working correctly for me lately either. It will launch correctly and idle for about 20 minutes at a time, and will then sign out and tell me I'm not connected to Steam (which I obviously am--solid internet connection at that). It's been pretty annoying to say the least.

I don't use Idlemaster but you can use TF2 and Trine2 to idle without actually install\run the game.
(TF2 and Trine2 give an error message, 0 resource wasted)

I haven't had that problem.

I'd suggest you ask in theit steam group. Maybe someone there knows the issue:

http://steamcommunity.com/groups/idlemastery

Resend a new code and try again?

Steam guard is interfering so shouldn't matter if its through a 3rd party program or not.
It be the same if you were trying to login using a browser.

This is the build I use & recommend...

https://github.com/MonkAlex/idle_master/releases

I had this problem! It was a real pain, because the code was correct but Steam refused it after many resends during an hour (almost 10 codes sent, all the same). The real problem was some leftover files left by a prior Idle Master version, before I changed my password and later updated to a newer Idle Master version (a clean install from v0.9 to v1.2). Just delete this folder found here:

C:\Users\<user>\AppData\Local\IdleMaster

Delete the whole "IdleMaster" folder and try again. Apparently Idle Master keeps trying to log-in with the encrypted old password.

Uninstall, make sure the install folder is deleted, and install the latest version.

I had the problem a couple weeks ago, and an update was released that fixed it.

If it doesn't solve the problem, and I'm certain that it won't, you have to delete all cookies on Internet Explorer. I know that you probably don't use IE, but every time it encountered an error, I just deleted ALL cookies from IE, and it worked.

haven't updated... so mine is fine last i checked (last week)

I moved to SAM because I had the exact same problem