When I put Maples Roots on my level 7 elf cleric, I am no longer able to play a "Ruby Demon Portal" because "Approval Denied - Equipment requires more talents". When I highlight any other divine item, all three items are green. But if I highlight the Maple Roots, the other two divine items go red. There are no error log entries. Also, when I attempt to take the Maple Roots off my character, all of the items in my mule disappear. They return when I exit and go back into a market.
It looks like I get the same error if I attempt to put anything else in the third divine item slot of my cleric.
Ruby Demon Portal is a level 4 adventure. If your any of your characters are above level 6 they are automatically lowered to level 6 just for that adventure. At level 6 none of your characters have talents to spend, but they may still have items equipped that use talents. You have to take these off and replace them with other items that don't require talents. Maple Roots is a level 9 item that requires a clear talent. It can't be equipped while running that adventure. The red glow is telling you that you can't equip that item in any of those slots (since you temporarily don't have any talents to use).
I think I understand. So when I have 3 item slots available and two of those slots have level 4 items, when I equip my character in the market with a level 9 item, then the reason the two level 4 slots are now show red when I hover over the level 9 item that I need to replace my level 4 items with level 9 as well?
Doing older adventures can de-level you, so you basically lose your talents for the run - thus you can only use items that don't require any talents. The lvl of the items doesn't really matter (apart from no item below 7 has any talent req) - it's just a power marker of sorts. Getting the red hover effect means it can't be equipped in that slot (due to the temporary de-levelling). Basically exactly what Zoorland said, but it seems you misunderstood.
