Too many parallel downloads for your plan
/limit page description for signed-in viewers: try again after about 5 seconds, or upgrade for more slots. Same simultaneous-download cap as the API account message.
Why this happens
HTML Accept leads to a 303 redirect to /limit when concurrency is full. The “5 seconds” line is friendly copy (aligned with retry-after hints), not a separate setting.
Diagnose and fix
1. Wait and retry, or finish an active download.
2. Upgrade to a tier with higher ul2_sim_dl_* if that is the product pitch.
3. Admin: raise Simultaneous downloads and confirm Premium plans are purchasable.
4. Guests get the guest /limit description instead.