Too many parallel downloads — wait until finished
Anonymous / guest API path when parallel slots are full. Product text: wait until your current download finishes. (Nginx may return a shorter “Too many parallel downloads.” that maps here.)
Why this happens
ul2_sim_dl_anon (often default 1) for IP-based guest locks. HTML guests more often see Too many parallel guest downloads on /limit.
Diagnose and fix
1. Guest: wait for the active download to finish, then retry.
2. Admin: raise ul2_sim_dl_anon, or encourage sign-in for more slots.
3. See Guest downloads limited at the same time for the guest /limit body.
4. Distinct from grant rate limits (“too many download links”).