Hello Adrien.
Using Akeeba, I restored my site from a backup taken before I updated Myplayer. With version 1.2.13 I found that the links worked with SEF enabled if 'Add Suffix to URL' was also enabled. This backup took me to a point where several extensions needed updating. I did them one at a time, clearing cache and checking results each step of the way leaving MyPlayer until last.
On updating to version 1.2.17 the original problems returned.
HOWEVER. I have installed a clean installation of Joomla 3.9.12 on the same server for testing purposes (
cleaninstall.apostat.es
). There are no 3rd party extensions installed apart from MyPlayer and here version 1.2.17 works just fine with SEF enabled.
So I don't think that the problem lies with MyPlayer and you have my apologies if I have taken up too much of your time. If however, you have any advice on how I should proceed I would be very grateful as currently MyPlayer is unusable on my production site which is due to launch on Christmas Day 2019.
Small additional - I cannot update my order number from admin.