I just spent like 6 hours on this. I was setting up an importer, and the data view was working fine. At one point, I added a join to test it out, then removed it.

Sometime later, I noticed that something was off. I could no longer view the data view of that table -- all I would see was a "user warning: unknown column 'feed_id' in 'field set'..."

What's more, the site path now showed up as regular/path/to/%F20Fview, with an extra percent-code for the forward slash (whatever it is). The path in the view itself was shown as "regular/path/to//view", and changing this didn't fix a thing.

Looking at the query, it was obvious that the table wasn't being inserted (no mention of "feeds_data_my_table").

After some poking around, I realized that there was a fishy property in the feature: 'join' => array()
I deleted this and recreated the feature, and everything began working fine. I can't be sure, but I think it worked fine for awhile after adding and remove the join, but something remained that was exported as part of the feature, so when a reloaded it (as I do every so often to catch little things like this), it started acting wonky.

Anyhow, hopefully this helps. I upgraded from ctools 1.7 to 1.8 at about the same time, but I don't think that was part of it.

Hopefully this saves someone else some time! No clue where to start in solving this bug though.

Might be related: #639606: Once a join has been created, it can not be removed or altered