Pretty sure I tracked down the underlying issue, or at least one specific reason that triggers the error.

If *any* of your characters on a server originally purchased the plot of land for an FC (any FC) and is still in that FC, any alts on that server will be unable to place a bid for an FC house (only the character that originally purchased the plot for the FC plot will be able to do so).

I was able to confirm this with a few people I knew who were having the issue. If they had a character that had originally purchased the land for an FC, that character could place a bid, but no other characters on that server could. However, as soon as the character left that FC, all of the alts were suddenly able to place bids.

I doubt that's the way SE intended it to work, but that seemed to be the "fix" - a character who purchased land on behalf of an FC cannot belong to that FC, otherwise all alts are locked out from placing bids.