Workaround for 4.x unable to upgrade to security release 4.2.8
From Joomla! Documentation
Reason for the 4.2.8 security only release was an issue with the WebService.
Websites on version 4.0 or 4.1 could be unable to upgrade because of different technical reasons. The workaround wants to make sure even pages with this problem can be safe.
This pertains only to Joomla! version(s): 4.0, 4.1
How to fix
If an upgrade to the latest Joomla version is not possible, deactivate all your Web Service Plugins and change important passwords like the one from your database. To find them all in your administrator area go to System -> Plugins and search for "Web Service" or use the type filter with the value "webservices".