I should have just been patient - coming back this morning, I've been able to post my message with no trouble at all.
It doesn't matter now, but if you're curious, I was getting the error message when I hit Submit. Just a kind of twenty-four hour flu, I guess?
I did clean out the board's cache last night. Maybe an errant bit of data in the permission system that was flushed out. We may never know for sure.