You can follow the stats of Steam here :https://steamstat.us/

your problem might be related to the fact that Steam servers quite sucks sometimes and are shutdown for few moments, making trading impossible for few times. however if your problem isn't related to that, The reason why you can't trade with a trader in particular is because he might have blocked you.

@Fallen : Thanks for you Answer!

Originally posted by Ete:

Hello! I really enjoy trading but one thing that keeps happening to me is the following : There was an error sending your trade offer. Please try again later.You cannot trade with ,,Username´´.(50) .... So i Googled the problem. However the result didn´t help (AlreadyLoggedInElsewhere = 50) Maybe someone can help ? :D

I strongly believe this is due to steam's cap on the amount of sent trade offers. I think it's in the region of about 30 trade offers. Hope this helped.

@Apeche : possible, because every time someone accepts my trade-> all pending trades get declined because of me not having the item anymore ... Afterwards i can send a lot more without getting the error.... [Sorry for my bad English, i am from Germany and its pretty late Kappa123]