Update :
I was so confused cuz i got the problem again. Then i downloaded the python api to see if it get the same error and i got no error...
I compared the requests made by the python api and my curl requests and i found the difference.
The Bearer wasn't the problem. I had to use "https://www.opensubtitles.com/api/v1/download" instead of "https://api.opensubtitles.com/api/v1/download".
I'm not sure why the subdomain "api.opensubtitles.com" results in a 500 Error on the download link for me.
Just wanted to share my experience if others have the same problem.