We have MediaWiki running on Apache but accessed via an IIS reverse proxy. The extension works perfectly when accessed directly however when accessed via the reverse proxy throws an ASP.NET application error.
Topic on Extension talk:Flex Diagrams/Flow
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I was able to get the diagrams to render by updating the value of $wgServer in LocalSettings.php to the URL that the site is being re-written to however editing via the re-written URL still throws an error and then display and edit at the direct URL no longer work.
Turns out the issue was the : in the URL - IIS by default does not allow this in the path - once I added the below to my config file and update my $wgServer to the IIS bound URL it all worked
<system.web>
<httpRuntime requestPathInvalidCharacters="<,>,*,%,&,\,?" />
</system.web>