I noticed the something similar with construction material from the logs at spawn, I had nothing on me, but it would only let me carry 1 at a time.
Rejoining fixed the issue. I'm assuming the server thought i was still carrying a siege ladder. I've seen this other times as well where you only have 2 slots being used, and you go to pick up a 1 slot weapon and it drops one of your other 1 slots. Rejoining usually fixes this issue, but this is slightly different from the original post.