I found out, that as soon water/fog boxes will be placed in a level, no mirror surfaces can be used, because graphic glitches appears.
How I tested it:
1) Created a portal by using the makeitgood "Erm, nothing to see here"-mode, where the mirrored surface should appear.
2) Created a rim-file by using the mkmirror tool.
Without water/fog-boxes in the level, the mirroring is working fine:
As soon a water/fog-box will be placed, there are graphic glitches.
The graphic glitch doesn't look the same all the time. Also, sometimes the glitch doesn't appear at all. At Singleplayer-races, the glitch appears much more often, then on Timetrials.
It makes no difference, where the water/fog-boxes are located. Also a very small water-box far away from the portal is causing glitches. (So there are definitly no overlaps).
Here are some screenshots how the glitches looks like:
It would be really nice, if this issue can be solved. So trackmakers would be able to use both (mirrored surfaces and water/fog-boxes) in one level.
Report issues and bugs here.