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Yeah, I feel you. I really do. Joomla!'s extensions installer and updater is in DESPERATE need of a rewrite. I have been saying that since a couple of months prior to the release of Joomla! 1.7 (when the updater was still a "new thing").

Unfortunately that's the only area of Joomla! where 3PDs are not allowed to innovate, punished with exclusion from the Joomla! Extensions Directory if they do. Had this not been the case I'd of course had already built a better installer and updater. I have the know-how. Perhaps I am the only person in the Joomla! world who has the necessary know-how to create an installer/updater which works across the vast spectrum of hosts Joomla! is supposed to run on. But instead of making it easier for users, the leadership is consuming itself in talks about using Composer and shit like that. As if Joe Average, the target audience of Joomla!, will ever care to replace parts of the CMS using an arcane command line tool. Meh, double meh and meh all over again.

As for your issue... The only thing you can do is go to Extensions, Extension Manager, Update and click on Purge Cache. Do that twice. If the list of updates doesn't go away, you need to delete the contents of the #__updates table (not the table, just its contents). Then go back to Extensions, Extension Manager, Update and click on Find Updates. If you get a blank page, you have a PHP timeout. In this case go to Extensions, Extension Manager, Update and click on Options. Set the "Updates caching (in hours)" to 5. Then repeat the previous steps. Repeat the Find Updates step as many times as necessary to stop receiving a blank page. Yep, welcome to insanity ;)