I no longer have access to the old account, all I have is a takeout file with more than 1000 subscriptions so I was really hoping I could use the takeout file importer functionality but I had no luck so far. I tried both versions of the extension, Chrome (on Brave browser) and Firefox (on Zen browser). In both cases, I see the dialogue to select the zip file, and as soon as I select the file, the dialogue disappears and - this is the unexpected part - the extension GUI also disappears. When I reopen the extension GUI after that, it is back to initial status with no channel IDs having been imported.

If anyone is able to offer some advice, I would be very grateful because I would really like to import subscriptions to my new account.