Collision messes up in certain areas of my level.

This area is an example. I'll keep running when I go into the wall, and I sometimes get stuck and can't move.

Oh, yeah, that can happen when certain areas of your level come too close to the Blockmap limit. Try shifting your entire level so that it's closer to the direct center of the Blockmap.

I tried that, and it didn't solve anything.

Perhaps if you provided a download link, so we could see the level?

I'll send you it in a Private Message; I don't want to hand out my unfinished levels publicly.

Hmmm...I don't know exactly what's causing your problem, but in the meantime, I found plenty of other flaws in your level that you ought to fix.

1. Needlessly high amount of sectors. You ought to merge any sectors that have identical heights, flat textures, and effects; it would help the loading time.
2. WAAAAAY too many vertices! I can tell that you really, REALLY want the level to look like it has smooth curves. But you could probably cut out half of the vertices, and the level would still look good, not to mention load faster.
3. I saw a few Unknown Thing Types. Any particular reason?

I apologize that I can't help with your big problem, but you ought to heed what I've mentioned until we can find a solution for it. It might simply be that the level is trying to render too much at once.

1. Some Sectors are separated because they have different brightness settings and other minor changes.
2. It loads just fine for me.
3. I placed custom Things in my level, and some of them have the wrong MAPTHINGNUMs.

I had the same problem at the end of Frog Forest V1. I fixed it, and the reason that was stopping me from continuing was really, really stupid. On that version I created a small mushroom (big mushroom, really, but not as big as the huge one). After that I couldn't progress at a part at the end of the level. I tried comparing the latest working backup and the latest glitched backup. I realized that it was the mushroom, that was at around 30% of the level's progress was stopping me from continuing at 97% from the level's end. I moved the mushroom a bit to the right, and nothing wrong happened afterwards.
Long story short: compare a working version with the current one and see what has been changed and what may be causing the glitch.

I reread your glitch, and it doesn't quite seem the same thing, but comparing backups is like a global answer. I just told my "story" to tell you to try anything. Something as stupid as a single set of sectors far away from the glitched part can make the difference.

That error tells you that your BLOCKMAP is not building properly. This can be very difficult to fix sometimes.

In what ways could I fix the problem?