The first possible solution, which is the easiest solution, is to go to your subscribed items for Unturned and unsubscribe to the map that you are trying to connect to.

Go to the default steam page
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Hover over "Community" and click workshop
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Type "Unturned" in the search bar and click on the auto fill option of "Unturned"
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Hover over "Browse" and click "Subscribed Items"
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In the list of your subscribed items, find the custom map you are trying to connect to and unsubscribe from it. After this, restart Unturned (if you had it running while doing this) and see if this fixed your problem.

If the first way doesn't fix the problem for you, this way will most likely work, but takes some more steps. If you are familiar with computers and browsing file explorer and don't want to read through the whole guide, here is the most likely path of where you will have to go (keep in mind, your files may be in a different place, but its unlikely):
C:\Program Files (x86)\Steam\steamapps\workshop\content\304930
Once in the 304930 folder, delete all of the contents in it and restart Unturned (if you had it running while doing this).

If you are not as familiar with file explorer, here is a more detailed description of what you will have to do

First, open "File Explorer" by typing that into the search bar or clicking this icon on your task bar
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Once in file explorer, click on "This PC" on the left hand side
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You may have multiple storage devices connected. You want to go to the storage device with the "Program Files (86x)" file in it (Most likely your (C:) drive)
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Once you have located the drive with the "Program Files (86x)" folder, open the folder
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Once in the "Program Files (86x)" folder, find the "Steam" folder (you may have to scroll to find it)
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Once in the "Steam" folder, locate the "steamapps" folder
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Once in "steamapps", locate "workshop"
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Once in "workshop", locate "content"
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Once in "content", locate "304930"
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Once you are inside the "304930" folder, delete all the folders inside. These are workshop files for workshops mods you are subscribed to in Unturned. Deleting them will not harm your game, they will all be redownloaded after you relaunch Unturned. After you have deleted the folders in "304930", restart Unturned (if you had it open while doing this). Once you launch unturned, it will redownload all of the workshop mods you are subscribed to and get rid of the error you got while attempting to join the multiplayer server.
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