Please help. There are a number of users experiencing this major bug. Please read: https://community.adobe.com/t5/illustrator/help-illustrator-2021-won-t-load/td-p/11530119?page=6

I think Adobe needs to realize that it has been isolated to Illustrator v25.0 only. Like everyone else, the latest releases of Photoshop v22.0.1, InDesign v16.0, and Bridge v11.0 all work just fine. Moving down to Illustrator v24.3 seemed to help get everyone back to work. However, since this is a newer version, there is a question on the projects that we have been working on with v25.0 will open up without any hiccups with v24.3. I am testing that right now as I have an iMac running Big Sur with Illustrator v24.3 and have a Macbook Pro running Catalina with Illustrator v25.0. So far I don't have any problems running v25.0 on the Macbook Pro under Catalina. In addition, I am running Extensis Suitcase Fusion v21.2.1 on both machines under both operating systems. I have been in contact with them as they are hoping to release a version that is supported on Big Sur. But when Illustrator v25.0 on Big Sur just bombed on its own, just like everyone else here (about the same time) I was able to somehow get it to load up into Safe Mode (don't ask me how I did it because like everyone else, I have seen Illustrator just sit there on the load screen regardless of it trying to initialize or do one of its many Diagnostic tests). But I was only able to get it to open in Safe Mode a couple of times. When it did, Illustrator identified that it had a problem with the Suitcase Fusion plug-in. I have since disabled the plug-in just for Illustrator through Suitcase Fusion and still, Illustrator will not load, even to a diagnostic state. But Adobe needs to understand that the simultaneous moment that multiple users experienced an abrupt shutdown of Illustrator and it not loading with the acceptable wait time, that there is something much more than "old residual files" present. There shouldn't be any reason that an Adobe employee needs to be inside monkeying around user's machines. Just like when the Create Suite was the latest product line before CC, it needs to provide a Tool Cleaner program to rid of all these "old" files for us now. Or during the large push of CC 2021, the installer should remove all instances of CC 2019 and CC 2020. (I just looked in my Applications Folder and found those folders for those previous releases. I removed them.) Please find a fix ASAP for us that have moved to Big Sur and want to run the latest version of Illustrator beyond v25.0.

I just tested the process of editing an AI file in Illustrator v25.0 and then opening it up in Illustrator v24.3. There doesn't seem to be any issues with the file opening with a legacy program. I edited the file on my Macbook Pro running Catalina and Illustrator v25.0 and then opened the file on my iMac running Big Sur and Illustrator v24.3. In addition, I have re-installed the Suitcase Fusion plug-in for Illustrator v24.3 and have no issues so far even though the iMac is on Big Sur.