Tiny MCE downloads files instead of opening (Source_editor, Link, Image)

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Hi all,

I'm updating a website of a client of mine, the website is running on version: 5.6.3.2.

And I wanted to edit the HMTL via the MCE editor but when I click on html or add link a blank popup will show and chrome(also firefox, edge, opera) will just download a file like: source_editor.htm or link.htm.

Does anyone of you have a solution for this?

See the attached screenshots,

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