LoadPortals: couldn't read C:\..\mymap.prt
The .prt file is a file that qbsp generates as a handle for being sure that the level you compile is free from leaks, that is a hole from the interior (play area) of your level to the great beyond outside your level's world. This hole is where some brushes aren't actually touching each other and sealing off. The hole could be quite small, or very large. You'll have to go back to your editor and find it.
Vismap expansion overflow
Only happens with -fast (Fastvis)
You can't run FASTVIS on big levels, just as we couldn't in Quake. There isn't a problem with your map, other than it's too big for a FASTVIS job. Used to be "leaf saw into leaf" or something in Quake map compiling with the -FAST tag.
Marc Pullen
WARNING: Leaf Portals saw into leaf
This quite possibly can come from two areas touching each other, when seperated by an area_portal or other vis-blocker. By two areas, I'm referring to two containers ( a room inside another room -- with both rooms having the SAME entrance, but their own walls, floors, and ceilings). The simplest analogy is to a shot glass within a larger glass. If the two glasses (containers/rooms) touch, even by a single point, it is possible to create this error.

It doesn't happen 100% of the time (since vis isn't ALL that predictable), and may not be the only cause of the error.

My tutorial on RUST explains what two areas are.
I have a room with windows that look out into a fake outdoor area (just for looks.) The ceiling, back wall, and side walls of the outdoor "room" are sky texture. I used several pyramid brushes (with moved vertices) around the bottom near the sky to make it look like a natural cliff. I tracked the error down to one of those brushes. The unique thing with this one brush was that the back side of the brush sloped down toward the floor. When I moved the faces around so the back side of the brush was perfectly aligned with the back wall (which was sky) so that there was no gap between the back of the rock formation and the sky, everything was fine.

I'm not sure I fully understand why this was the case but hopefully as a community we can figure all this weirdness out.
fastvis = true
reading c:\quake2\baseq2\maps\MARTY4.bsp
reading c:\quake2\baseq2\maps\MARTY4.prt
2436 portalclusters
8244 numportals
************ ERROR ************
LoadPortals: reading portal 2955
error leaf with too many portals
This happens when you have a small area with a very high amount of detail. The solution is to simply find that area in your map and either simplify or rebuild it.