If you are using Google Drive with shared drives (including any G Suite drives except the one owned exclusively by your Google user account) you will have to set up a self-hosted OAuth2 helper in one of your sites, and use it with your backup profile's configuration. This is due to a change on Google's side.
New style for the Backup-on-Update information. In previous versions of Akeeba Backup we were using Joomla's system messages to print the notification for the Backup-on-Update feature in Joomla Update's main page. Unfortunately, Joomla! 5.1 introduced a number of CSS changes which adversely affected the legibility and layout of this message. Starting with this release, we are now using a different approach where we inject a block of HTML right above the Joomla Update component's output.
Alternate Configuration page saving method which doesn't hit maximum POST parameter count limits. Our backup profile configuration page has gone over 100 individual configurable parameters. When this is submitted back to the server, it would trigger the maximum POST parameters limit in a stark minority of servers which expect 100 or less of these parameters. We are now using client-size JavaScript code to serialise the configuration to JSON before submitting it to the server as a single, fairly small JSON document, therefore overriding the observed issue.
Improved support for Joomla! 5.1's backend colour schemes. Joomla! 5.1 introduced support for site-wide, and per-user colour scheme selection i.e. if you want to always display your site in light / dark mode, or if you want the display to follow your browser's preferences. In the past dark mode was only controlled by browser preferences. This created a situation where the browser preferences may indicate one mode (e.g. dark) whereas the site-wide or user setting might indicate another (e.g. light). This led to some display artefacts in our software, such as text becoming hard to read or completely invisible. We have now addressed these Joomla changes in our CSS.
Bug fixes and minor improvements. Please take a look at the CHANGELOG below.