Could you try using a different browser or clearing your cache? Just so you know, Composer is still in alpha, so it might not work properly right now.
If that doesn’t help, here’s how you can share the HAR file with us:
1. Open the login page, but don’t send any messages yet. Open the developer tools.
2. Click on the network tab, enable the recording icon, and make sure to check "preserve log".
3. Go ahead and try your composer attempt.
4. Once the attempt is done, export the network logs as a HAR file and send it in your reply here.
This should help us get a better understanding of the issue you’re facing.