I’ve received similar results for Doppelgangs around 5, when I had multiple mana-duplicating effects active.

Can you, at least for now, make it so you don't have to select all cards (without removing the limit when there aren't enough targets)?

What i mean:
If there is enough mana but only 10 permanents the maximum X can be is 10 (because there is no 'may' in the card text).
So if you pay X=11 you need to select 11 permanents, else you can't pay that way.

I had games where i won by copying a land 20 times that deals 1 damage to target oponent, but because I had to select 19 other permanents crashing the server I got a draw. If I only would've targeted the land it wouldn't have crashed.

I am aware that the forced targeting makes you target permanents that have may have negative impact on you (e.g. enchantements with draw a card take 1 damage copied 20 times), so it is a greed/awareness trap.

But i think being forced to target all permanents and crashing the game is a bigger deal than not losing because of that trap.

And being forced to lessen the potential of 'Doppelgang' by not having X become big is even worse.

Same here. So after researching the GRE.error and comparing it to "horn of gondor" together with "erkenbrand", "theodin" and "impassioned orator" which can trigger a similar amount off effects but doesn't require the same amount of mana, doppelgang seems to break the part of the Game Rules Engine that is responsable for autotapping mana to pay for spells. I will now try if it works in full control when I tap mana myself.

edit: x=13 seems to work by paying manually (in a bot match)
edit2: x=20 seems to work by paying manually (in a bot match)

edit3: x=20 didn't reliably work in real matchs.

For me it seems to happen whenever I Doppelgang for enough to exceed the token limit in one go (eg I do it for x=6 and then for x=15, the second one hangs and causes a GRE draw)

I've received the same results from doppelgang's as small as X = 7