This error means that the game cannot locate the build (latest patched game files) in the unusually directory.
Please check if you have a folder called 'Production' or 'Testing' in this directory:
Windows - 'C:\Users\<yourUser>\Documents\RealmOfTheMadGod'
The issue might be caused if you have spaces or other special characters in the path to these files, specifically the <yourUser> part. Make sure that the user name you have in the path does not contain any special characters.
Next, if you have these folders - first make sure you have uninstalled any previous versions of the Exalt client and then delete both of the folders and try to install and patch the client again. Should that fail, delete them again (if they were even created this time) and try switching between 'Production' and 'Testing' environments in the Exalt Client before attempting to patching again.
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If you don't have these, uninstall the game from Steam, try to download the game launcher from here - https://www.realmofthemadgod.com/ , create a temporary ROTMG account with an email address, if you don't have an email associated account already and then log into it. Once that is done you should be able to update the launcher and this will place the latest build in your documents folder. Now install the game from Steam again, and try starting the game from Steam to see if that solves the issue.
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Alternatively, you can remove the Production and/or Testing folders, uninstall the game, create a new Windows user account with full privileges on this computer (with a name that doesn't have spaces or special characters) and install Exalt again on that user to see if you can download the patch and play the game.
If that works, then you likely need to rename the user folder for the original Windows user account.
Here is the link, this article is from Microsoft so it's as official of a workaround as can be, but we have to warn you that our team cannot take any responsibility if the instructions in this article cause any potential issues with your computer.
https://learn.microsoft.com/en-GB/troubleshoot/windows-client/user-profiles-and-logon/renaming-user-account-not-change-profile-path
Make sure you rename the original user account folder to something without spaces and/or special characters. Only use A to Z.
Please note that if you do not feel comfortable fiddling with the registry in order to fix this, then the other workaround remains to play ROTMG from the other Windows user account or to try and migrate the build folder from the new to the original user account documents folder.