Not sure MountainClimbing.mbz is really any good for use cept testing restore of a course and what could go wrong. I had no issues with it. But couldn't use some content contained there in and noted below (for what that's worth).
Files date from Mar. 6 2013 ... over 7 years ago.
No offense to authors ... example: one question contains a .wmv video - Windows Media Video ... which is a proprietary video format and probably seldom used in any modern moodle or web site ... unless a Windows Bigot!
Ditto for a Snow_and_Ice_on_Pinnacle_Ridge.3gp found in course. Who uses a .3gp for Moodles these days?
Ditto for La haute montagne avec Gaston Rebuffat.flv ... a flash file ... even Adobe (the maker of flash) no longer supports.
Shall I go on? (not!)
'install some themes' ... are they compat with your version of Moodle?
'having difficulties uploading a video' type of video?
Did you check Site Admin Menu -> Server -> php info to see if the changes you made to php.ini for file size uploads really did take? (I've asked that before but no response).
If your moodle is a tinker instance, fine ... fool around with it. If you plan to use it for real courses, time to do some planning and put something usefull up - not junk that you will/should clean up later.
Another question ... related ... did you set up a cron job and is it successfully executing every minute ... yes, that's the recommended frequency now ... once every minute.
Site runs nginx so it has to be Linux of some flavor (nginx doesn't make a version for Windows) so what flavor of Linux is it?
Where to look ... first, these failed restores of backup are probably taking up some space in your moodledata/temp/backup/ directory. Manually remove any files/folders in there. We have no idea of how much space you have left on your server and the last thing you want to do is run out of space!!!! Database (assuming db server on same machine) will get corrupted when one runs out of space and difficult to repair.
Look in nginx server error logs for clues or other error logs for clues.
Or, turn on debugging all the way to developer and let's see what Moodle can tell you.
Look over https://docs.moodle.org/38/en/Nginx
Sounds like you are a beginning server admin ... Google is your friend ... If Moodle HQ had a true blue server admin course I'd send ya there but, alas, there isn't one that covers it all.
If anything really does 'jump out at you' in logs ... DUCK! ROLE! AND RUN!
We all pick our own 'poisons' ... and then learn our own antedote.