Control panel -> Programs and Features -> MS .Net framework

What version do you have installed then? I have 4.6.2 installed on win7. If it is installed then go Programs and Features, right-click it and repair.

win10? Get a later version.

Repair Microsoft .net framework, ensure version is 4.62 or more

Uninstal it. Delete it's appdata. Download the Zip version 1.4.1. EXTRACT it.
Log out steam.
Start steam, start idle master. Log steam in 1st.

See here:
http://psteamcommunity.yuanyoumao.com/groups/idlemastery/discussions/0/

Idle master reads your badge page, the page you see here, it looks for 'card drops remaining'
http://psteamcommunity.yuanyoumao.com/my/badges

Steam gives the cards (not idle master), steam only ever drops half a set of cards.
http://psteamcommunity.yuanyoumao.com/tradingcards/faq

You need to buy the rest or trade for them:
http://psteamcommunity.yuanyoumao.com/market/

Free-to-play games do not drop cards unless you spend lots $$$ on in-game purchases or DLCs

Idle master reads your badge page, the page you see here:
http://psteamcommunity.yuanyoumao.com/my/badges

Steam gives the cards(not idle master), it only ever gives half set number of cards.
http://psteamcommunity.yuanyoumao.com/tradingcards/faq

You need to buy the rest or trade for them:
http://psteamcommunity.yuanyoumao.com/market/

Free-to-play games do not drop cards unless you spend lots $$$ on in-game purchases od DLCs

You have MS .Net 4.62 or better? Might need repair.

Instead of restarting Idle Master, have you tried pause/unpause or changing farm order settings?

Sometimes I think steam gets bitchy if you have too many simultaneous sessions going like: Steam, Idle master + 2 or more internet browsers etc. The more sessions you have, the quicker steam logs you out of one of them... maybe.

Might be steam needs an update, might be network changed IP or temporarily lost connection, you may just need to log out of steam and idle master, close both, restart them, log in steam 1st then idle master.