GPU Memory is Full when media Transcoding

In Media Management, just for proxies. No grading, no Fusion...

You need to tell what is your machine spec (you have few in your description), project settings, source media etc.
Based on your current description we can only say "that it may rain tomorrow"

Cineform all the way.

Still same answer: "maybe it's going to rain tomorrow".

Windows 10 Pro, GeForce GTX 690 2Gb
Transconding 3840x2160 (Canon XF-AVC Intra) to 1920x1080 (project's resolution)

Most likely related to fact that GPU has only 2GB RAM, but your source files are 4K. It can be also a bug, which there seems to be reports about.

I'd also suspect the GPU.
The originals need to be read in source resolution and 4K is not double, but four times the data. To find out, you may try to generate optimized media.

The originals need to be read in source resolution and 4K is not double, but four times the data.

Still 4K image is less than 150 MB, whichever way you look at it (RGBA, 32bit per channel). All these GPU memory full errors that come up in numerous threads sound to me like bad GPU buffer management. When transcoding intraframe codec from single source, only one frame has to be in memory, if at all (if decoding-encoding is done on cpu, there is no need to upload frames to gpu).