Importer hangs infinitely on a (GUI Sprites) Texture modification (U19.2.5f1)

Hello

I'm using Unity 2019.2.5f1 on MacOS.

I use a small plugin, Simple Sprite Packer, that allows me to create atlases in a way similar to what was done in NGUI (iirc): you drag and drop textures into an interactive box to add them to an atlas.
Despite some warnings about this and that being obsolete, this plugin works perfectly.

It creates two files, a texture manager that is unique to this plugin (let's call that the SPP file), and an atlas texture it points to, a traditional Texture with 2D/sprite mode.

I verified the extension again in an empty fresh project and it works. The atlas, built from the same set of 2D textures I need for my game's UI, is generated very quickly and modifications are managed rapidly too.

For some reason, in my current project, a file which had no issue beforehand is now causing me trouble.

First of all, simply doing a reimport on the GUI atlas (texture file) takes quite some time. Applying changes too.

However, the real problem comes when I modify the atlas from the SPP file, either to remove or add a sprite to the list, it loops infinitely (beach ball of death).

I'm forced to quit Unity. When I reopen it, I get the following message about the atlas texture:

I attempted many things to solve this problem, from using an old version of the SPP and atlas files (which for all intents and purposes are identical in their sprite content), deleting meta files, importing/reimporting either the current one or the old one (from the archive) within Unity, or copy/pasting it through the Finder, to see if there would be any difference.
Nope, every time I tried to edit the atlas through the SPP file, it keept looping on and on.
Again, the SPP plugin works flawlessly, I can create any new duo SPP-atlas without any trouble, but THIS current atlas is doing some nasty things.
There were no issues on previous versions of Unity (2017, 2018 and even an older 2019), but since I didn't need to change that file in a while I didn't notice the bug.

I've read some other threads, vaguely similar in topic, but they are few and some people talk about manipulating the Library folder, which I'm just not eager to do at all. I mean, how safe is that?

An option here is to blast this current GUI atlas and its relative SPP file and create a new SPP-atlas duo, but then ALL sprite references, either as fields in graphic components or in scripts, will be lost. :-(

Another option would be to "update" Unity and hope the bug doesn't happen, but that's too speculative and it would take time, and also eat more space on my computer, which has very little space available right now: I suppose it will import the iOS/Android modules again and that's the last thing I need right now, since it appears any "update" is actually an entirely fresh installation and I'm not sure if two fairly recent Unity installations can point to the same bunch of modules or if they need their own versions (I'm going for the second idea).

 

I forgot to add that if I open the project after the forced quitting, the moment I click on the atlas and then select another asset, I'm asked to apply changes or revert, so I check the current params, apply changes, wait, but the changes are not applied as I'm still prompted to revert or apply changes.

As a matter of fact I just noticed that trying to apply changes to new UI elements, before adding them to the atlas, is impossible too. There's the loading bar that shows all PNGs being processed, but then I still have the revert/apply buttons still active.

There seems to be an issue with validation of changes and perhaps the implied refresh/import that might occur right after that (is that it?).
When I deselect all files, I'm again prompted to revert or apply, I do cancel then select the files again, but then the changes are applied.

Sidenote: Is it normal if I cannot import an entire folder into my assets and must use the Finder instead? Unity only accepts separate files.