I hosted a map last night with the Warehousing mod 0.2.1. Everything ran wonderfully. A second player couldn't get on the map. "mod not available in mod-portal". Further he could not do anything about it. But the mod was installed correctly. Tried several times.

Then I started the map without the mod and everything ran correctly. The 2nd player could join. It seems to me that it was due to the mod. Could you please check that?
Many thanks and also for the mod, which I learned to appreciate very much.

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Obviously the mod is on the mod portal, or this discussion thread couldn't exist.

This sounds like a Factorio bug. I ran into another mod yesterday that said "Not on mod portal" when syncing (to a local save); possibly the issue affects more than just deprecated mods: https://forums.factorio.com/67003

That could be it. The mod is and was not outdated. That's why it's all the more inexplicable.