Can't upoad .tcx file to plan program

Hi, I wanted to upload a .tcx file to setup a training program.

I am pretty sure I uploaded this file back in either December or early January, but later cancelled it and now when I try to upload it again, I get a “Error: Something went wrong.” pop-up. In the browser console, the server response is a 500 with the response message “Server Error”. I tried all this multiple times, on 2 pcs with different browsers and/or different operating systems.

The file is manually created and exported from Strava.

Is it an activity recording or some other tcx file?