I just found that my Garmin Colorado 300 with firmware 2.6 rejects caches downloaded via Garmin communicator 2.6.1
Those caches appear as waypoints, but not as caches. The same cache downloaded directly as GPX, and then transfered to Colorado works fine. The problem seems to be in <time> tag.
In GPX downloaded via communicator it looks like:
<time>2007-09-29T00:00:00.0000000-08:00</time>
And in GPX downloaded direct:
<time>2007-09-29T00:00:00</time>
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OH3MBC