When a registrant starts the form process, they are placed and tracked in the system's memory.  This tracks and reserves any inventory they select while they are incomplete and prevents overbooking.  If they stop going through the form and that time limit is reached, the items they selected (agenda items/sessions) are canceled and returned to the inventory pool.  As they take up system memory, there is a limit on the number of spots available for each form.  This can range from 150 - 900 slots based on the current system resources.  Once this limit is reached, registrants will see a notice on the form that it is unavailable and if they wait it will check every 10 sec or so to see if a spot has freed up. With the time limit set at 5,000 minutes, if an incomplete reg does not complete the form, they will take up one of these spots for 83 hours.

The system default of 10 minutes is normally enough, and if more is needed, increasing it to 30 or 60 minutes is often all that would be necessary.  When setting the time, you need to weigh how long you think a reg may pause in the form, how long you want that inventory to remain unavailable to others, and how many people you expect to be registering in that same timeframe.  The higher the time you set, the longer the system allows an incomplete reg to pause on the form but limits the number of regs that can access the form at the same time. The higher this is set and the more people you expect to register, the more likely you will run into this error that prevents people from going through the form.

This has been the behavior of the system for a very long time.