Can't reproduce this. I added code to show the barrier zone and there is no gap, at least on my machine.

It seems doesn't appears often. It also randomly appears on right side or left side of the arena gate. I tried five unmodded run and it only appears on first run. Then I tried playthrough with Godship mods which allows me go to Rigel Aurelius quickly. This gap only appears in last out of six playthrough.

(See thread forums for the images, I don't know how to add image in here)

To check if the gap bug appears in the arena, shoot a weapon between the wall and the gate. I suggest to use spreading weapon such as TK10 Trident cannon or laser cannon array, but don't use cloud weapon like PK25 Morning Star cannon because it won't works.

Here also the savegame: https://www.dropbox.com/s/crxvo8bnuqwlnte/DigaRW-1854.sav?dl=0

The game need Taipan GodShip mods from relanat, download from here:
http://xelerus.de/index.php?s=mod&id=1599&d=1

Could be round-off error. Depending on the exact location of the Arena, the segments might get placed 1 pixel off. That would account for the gap. I'll look at that.

Found the largest hole of my life when my arena opponent just left through it, here's some video of me flying through it: https://youtu.be/JSLk2UyrOyQ
Not consistent between saves, started up another save with the same mods and didn't have any wall issues.

These are my latest hypotheses:

• Round-off Error: Maybe an arena segment (like the door) is getting placed 1 pixel off because of round-off error (when converting from kilometers to pixels). But the video that @puffo12346 uploaded shows a very wide gap—much wider than a pixel—and the arena images look correctly aligned. Status: Disproved by video.
• Hit-Mask Missing: Maybe the hit-mask for the Arena is failing to load (due to memory problems or bugs in unzip code). But if that happened, then none of the arena collisions would work. Status: Disproved by reported symptoms.
• Hit-Mask Corruption: Perhaps the mask is getting corrupted on load. I tried a stress test loading the mask, and it showed no problems. Moreover, the video shows that the mask works almost everywhere except for a one (maybe two) places. It's hard to imagine limited corruption like that. Status: I can't fully disprove this hypothesis, but I find it unlikely.
• Door Switching: All the reports I've seen show problems around the door. Perhaps there is a bug when opening/closing the arena doors. Status: Possibly related.
• Collision-Test Code: There could be a bug in the collision-testing code. Maybe it's getting confused when the door opens and closes. Status: Need to investigate.

Found it! I loaded @digarw's save file and I could reproduce it.

The problem was that the grid zone I use for collision detection was too small. The Arena door is so large that in some cases it was outside the grid zone relative to the player. I.e., it was never getting checked for collisions.

It depends on how the Arena is positioned relative to the grid. Most of the time, the door in the same grid square as the player (and thus collision works). But sometimes, the position is such that the player is on one grid square and the door is on a neighboring square, which does not get checked.

The fix was to increase the radius of the grid to check.