I've been suffering from this problem and tried all the fixes suggested in this and other threads. I've use a seamless tile effect in my image editor (PSPX4) so the 1 pixel trick doesn't work full stop.
What I did eventually discover was that if I imported the object at exactly the resolution I use it in the game the tile border effect disappears and the tiling becomes truly seamless.
This may be a bit of an obvious fix to many but I was importing at 128X128 then resizing in the C2 image editor to 64X64. Importing at 64XX solves the problem on all the tile types I'm using. If all else fails I suggest trying to resize the affected tile in a 3rd party image editor.