I am not sure these repro steps will work since I don't have the items anymore to try it again but here is what I was doing to have the issue occur. I am also unsure if this was the actual cause either but the guild stronghold coffer was the solution to fixing the issue.
1.) Go to a guild stronghold and open the coffer window
2.) Have a stack of an item you can donate such as [Spoils of War] into a surplus equipment coffer that is nearly full. The coffer will need to have enough space to hold 1 Spoil of War but not a stack. (This may have been donating to an alliance instead of a specific guild)
3.) Donate the item and get the error message that donating the goods would put the coffer over the limit.
4.) Leave the guild coffer
5.) Attempt to buy any item from any vendor that is stackable and find the A Confirm button is greyed out and unable to be used.
6.) Try to log out and reset the game to solve the issue, No avail
Expected Result: The ability to use merchants/campaign windows
Actual Result: Inability to buy anything stackable, Sell Anything Stackable or Claim Campaign Rewards from New Sharandar Campaign
I was able to fix this issue only by going back to the coffer and finding an item with a minor enough value to stuff into the surplus equipment coffer. After doing that my campaign rewards came in and I was able to buy/sell items to merchants again.
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