https://www.afkmods.com/index.php?/topic/10531-ussep-navmesh-errors/
A large number of recent USSEP 4.3.6 errors (the current release), in more than one navmesh. See listing at topic. Affects [00025F48], [00025F49], [00025F4B], [00025F5C], [00028D15], and [000F43EF].
Some fairly strange things, such as a new "island" inside [00028D15], sharing vertices with adjacent triangles, but not linked to them. Numbered 13, a very low number. Somehow, the CK made a serious error!
There are so many, it was easier to copy the originals, and painstakingly examine what kinds of things were done to them for USSEP using the indispensible SSEEdit, then do them similarly without generating errors.
A floating navmesh island [000F43EF] was in the air near the foot of a toppled Centurian, USSEP grounded it on the plinth, I've added some triangles in a pie shape, and 3 edge links down to the floor. Now followers (and enemies) can jump onto the plinth and snipe.
Added up/down edge links from long path [00025F49] to navmesh island [00025F4B], where a chest and body are found. Fixed that navmesh, as it was under the chest and body.
There are more navmesh islands, but they are either under ice or under water, so left them alone for now.
Rated critical because it's a USSEP error, and no follower can pass without the engine teleporting them.