G’day, I have seen this pop up from time to time and have recently faced the problem myself:
I had 8 pages of bookmarked maps, and only 4 pages would load → when I proceeded to the next page, it would not load at all and would say that it is not available
What I found resolved the issue for me:
Logging into Halo Waypoint and unbookmarking all of my maps, gamemodes and prefabs
The page wouldn’t load all ‘144’ results, so I proceeded to un-bookmark all of files that were loaded up → then refreshed the page to continue following the same method; until everything was un-bookmarked
As a result I am now able to bookmark and read all of my pages again 
The source of the issue:
From what I have read from others, it is a matter of a corrupted file / someone un-publishing a file (map, gametype or prefab); this can be seen when the file has no name/title*
For me it was a prefab with no name, and this also affected my bookmarked maps when I would attempt to load the following pages
Hope this helps! 
Would love to hear if any of you guys have any amendments / other solutions too*