same
This is the console error:
[Advanced Duplicator 2] lua/advdupe2/sh_codec.lua:458: malformed dupe (wtf <5 chars long?!)
1. error - [C]:-1
2. Decode - lua/advdupe2/sh_codec.lua:458
3. InitializeUpload - lua/advdupe2/cl_networking.lua:243
4. DoClick - lua/advdupe2/file_browser.lua:417
5. OnMouseReleased - lua/vgui/dlabel.lua:232
6. unknown - lua/vgui/dmenuoption.lua:77

It's a conflict, I was in the same server as TheOfficialTini and despite having all of our add-ons in common, I didn't have this error, but he did, and he has some more addons than me (all server-side addons that I didn't need), so it might be a server side addon like ULX, but we are yet to test this. (i'll do it in 5 mins brb)

Tests:
Results-None

It's the addon, but still strange it works with TheOfficialPascuttini.

Incredibly strange...

Not a conflict and apparently.. not the addon?

I got a theory: My garry's mod has fewer bugs than TheOfficialTini's (Gmod bugs like turret and rollermine staying as "ragdolls" after dying, thus glitching) for some reason. so maybe its an issue between garry's mod and this addon internally?

basically not something you just fix.

I'll try the official github version to see if its any better

This (may) be the issue I was having, I no-longer have it but I believe my issue was either due to lag when opening a file or because some of my files in my advdupe2 folder were not the correct file. (I mess around with E2 alot and some of the E2 files I had made were to copy and paste onto a chip instead of a dupe of the chip) so it may have been breaking due to unreadable files.

So the dupes easily corrupt with E2? could be something, but despite of this, me and TheOfficialPascuttini have similar dupes saved, and some load on his computer, while don't on mine... could be a performance problem maybe?

You were right! (and its not the E2s btw)
It was a corrupted file (which was infact buggy as itself, and when svaed, it corrupted all Advdupe2 so yeah, if anyone is getting this problem, try erasing some dupes, one might be corrupted.

Issue solved lol, It took me multiple hours searching for a solution to find none, Now people will find this. ;)

Thanks for giving a solution to this, I could just help out a peer.