UPDATE: Here’s the official info from Zenimax community managers (source):
Last night a few North American users reported inventory-related issues with their character account, such as seeing additional inventory items appear, or seeing items that were not earned through gameplay. We’ve identified and fixed the problem, but the issue necessitated some downtime overnight for the North American megaserver. This problem has not manifested on the European server, so it continues to run as normal. The fix will be implemented as soon as all required pre-testing is complete this morning.
We were already planning to push a new game build to both servers this morning, and we will continue to work towards this. Stay tuned for complete patch note shortly. In addition to this aforementioned priority fix, we are also working quickly to restore our ‘send in-game mail’ functionality, some content blocking fixes that are preventing some users from finishing quests, and a few other tweaks. Our goal is to have all of these fixes initiated later this morning, eastern US time.
We will update this post as soon as we have an exact timeline for the service to be restored.