Hi! This is a known issue that we are tracking internally. If you'd like to send us a copy of your game log from a play session where this has occurred, I can add it to the relevant report (details of where to find it are in the FAQ thread - keep in mind that the log gets overwritten each time you start the game, so be sure to grab one that includes this behaviour).

Logs can be popped up to somewhere like dropbox or pastebin and linked here, or you can email them to support@defiantdev.com including a link to this thread.

In the meantime, please avoid taking the Armour of Gluttony on the Strength challenge.

Apologies for the inconvenience!

I didn't take a log at the time, but I shall if it happens again.

No worries, the game came out pretty recently, a few teething issues are to be expected.

Happened to me too yesterday. Couldn't upgrade it in two separate Strength challenges. In the second challenge I got the armor at the very start and couldn't upgrade it throughout the whole challenge, neither with the challenge-specific blacksmiths nor the player-owned blacksmith. But I did have a lot of food that time and I saw one forum member say that if the armor's ability is activated (which happens at 20 food), then it won't let you upgrade it. If that is the case, then the solution is to eat some food (bring it down to 19) and then upgrade. Will try it if I get another opportunity.

Edit: I did 2 more Strength challenges and in both it allowed me to upgrade the armor, but I had less than 20 food.