After a quick search i found this is almost always caused by a leak in your map or 3dskybox, but i could find no leaks.
Long story short, i found the problem to be a "phantom" func_illusionary, i.e., there was no visual brush, at any of the coordinates it gives you, but when checking the entity report, i found 3-4 func_illusionaries that when you selected them, there would be simply nothing there. With these "phantom", invisible brushes selected in entity report, i deleted them.
After deleting them, map compiled fine! This seems very similar to hammer 3.x "func_illusionary/func_wall was empty" error, yet Alt-P didn't catch them.
Thank god i noticed the part of compile log, saying that an illusionary was causing the problem:
fixing up env_cubemap materials on brush sides...I was seriously about to slit my wrists trying to figure out what could possibly be wrong.. Now i'm happy.
0...1...2...3...4...5...6...7...8...9...10**** leaked **
Entity func_illusionary (-4854.73 101.47 -472.00) leaked!
FindPortalSide: Couldn't find a good match for which brush to assign to a portal near (5736.0 -3072.0 418.5)
Leaf 0 contents:
Leaf 1 contents: CONTENTS_SOLID
viscontents (node 0 contents ^ node 1 contents): CONTENTS_SOLID
This means that none of the brushes in leaf 0 or 1 that touches the portal has CONTENTS_SOLID
Check for a huge brush enclosing the coordinates above that has contents CONTENTS_SOLID
Candidate brush IDs:
