I recently returned to D2 classic, playing some single player and some online, with loads of nostalgia. Also remembering the main headache - running out of room. In single player I have been using ATMA's stash feature to dump items onto, saving up enough to build a full set, and twink alts. But I keep running into occasional items that I can transfer to the ATMA stash, but then cannot transfer back. So far these have all been set items. Sigon's Guard is the most recent example, but also several Berserker items (not the axe). I literally moved the Sigon's shield to stash then immediately tried to recover it without luck. The circle with a dash through it icon when trying to drag, no error message. I googled, read forums, tried all the ATMA options I could find relevant but was unable to fix it. I saw others mention this problem over the years, but no fix, no cause.
Through my playing about I think I may have figured out the cause however. So I am posting in case it's useful for others. The only thing I have been successfully able to do with these non-recoverable items is move them into a new character created via ATMA. I can then move them back and forth from that character and stash, with no problems. When I load that character in D2 (the game) it announces that this is an expansion character. So I suspect that certain items are considered to be expansion items by ATMA, and can only be moved to expansion characters. The same non-recoverable items are also notable in that I wasn't able to transfer them from one character to another, before having moved them to the ATMA stash. In other words it isn't the stash that converts them to expansion, but rather the item itself is considered by ATMA to be an expansion item. Move it from the character and you won't be able to recover it. I don't know of any way of bringing expansion items to classic characters, so this isn't a fix but more of a prevention.
Through my playing about I think I may have figured out the cause however. So I am posting in case it's useful for others. The only thing I have been successfully able to do with these non-recoverable items is move them into a new character created via ATMA. I can then move them back and forth from that character and stash, with no problems. When I load that character in D2 (the game) it announces that this is an expansion character. So I suspect that certain items are considered to be expansion items by ATMA, and can only be moved to expansion characters. The same non-recoverable items are also notable in that I wasn't able to transfer them from one character to another, before having moved them to the ATMA stash. In other words it isn't the stash that converts them to expansion, but rather the item itself is considered by ATMA to be an expansion item. Move it from the character and you won't be able to recover it. I don't know of any way of bringing expansion items to classic characters, so this isn't a fix but more of a prevention.