Follower's inventory doesn't show up for trade

Solution 1:

What i did is, I opened the console, clicked on Mjoll and typed 'removeallitems player'. With this, I removed all of her items and added the to my inventory (so I don't lose anything I had given her).

After that I spoke to her and told her she was naked. She immediately wore her default armor. After that I could be able to trade with her again. I don't know which part of this process solved my problem, or if it was random.

By having her stuff, she had her default armor in 5 copies (cuirass, helmet, etc etc). This is due to a bug in the game where each follower, if dismissed, and aquired again, will add the default items in his/her inventory (instead of adding just the missing ones, or the ones that there is no replacer). If you dismiss and aquire the follower 5 times, he would have his initial armor five times in his inventory.