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Hello. responding to your inquiry about Forge maps that you posted on my profile awhile ago. Sorry been so long... I been off of Steam and any its games for awhile, taking a break playing other game that is not on/associated with Steam. Only about 2-3 weeks ago, have I been back on Civ V, long enough to finish one marathon game, now about half through another. In all this time playing Civ V, I never once considered any activity on my account here on steam, til today.

About your question... I remember something about trying to play games with friends in the past, and so long as played on vanilla maps, seems to connect and work just fine... I remember once or twice trying to play on MP with custom map with friend, and having error messages similar to what you stated, but I don't remember all the details around the circumstance. So unfortunately, I can not help you, I'm sorry.