So yes, it is caused by the "Solve captcha on server" checkbox which is effectively using Microstock.Plus for automatic authentication instead of solving captchas manually in the built-in browser.
Since the addresses of our servers obviously differ from your local computer's address, the agencies websites are going to complain and send SMS to verify that it's really you.
The most paranoid and retarded in my opinion is Shutterstock which is forcing you to pass a password reset if you want to log in from a new address and there's no way around that.
So you can either let M+ to pass all those procedures for you (it will need codes, etc) and then the agencies will stop complaining OR disable the "Solve captcha on server" checkbox - that way you would have to solve captchas manually but the websites will not complain much because everything will be done from your local IP address.
In any case, the user has contacted me and I've helped him to disable all agencies in M+ and make sure no agency is complaining anymore. Hopefully at some point they will give another try to StockSubmitter to optimize their workflow
Also we're planning to implement two features that will help with authentication without going through so many hassles. Stay tuned