What is the latest available version of the stable release of CCleaner?
The release notes at https://www.ccleaner.com/ccleaner/version-history currently shows that v6.14.10584 (rel. 19-Jul-2023) is the latest version, and this is also the version shown on the Download Builds page at https://www.ccleaner.com/ccleaner/builds . However, if you click the Download buttons for the Free and Slim installers on that download page you will actually be offered an installer for CCleaner v6.15 (i.e., ccsetup615.exe; ccsetup615_slim.exe). Note that the Download button on that page for my CCleaner Free Portable is still offering the ccsetup614.zip file released in July.
I don't know if someone at Avast / Piriform has accidentally posted v6.15 beta versions of the .exe installers on the download server before they were ready for wide release, but I only noticed this discrepancy because my SUMo (Software Update Monitor) utility has been recommending that I update to v6.15.10623 for at least two days now.
If I recall correctly, it's been a few years since the version of the Portable build at https://www.ccleaner.com/ccleaner/builds (e.g., ccsetup6xx.zip) lagged behind the version of the .exe installers. Hopefully it's just another oversight on the part of Avast / Piriform and doesn't mean that the release of ccsetup615.zip was delayed or pulled because of (yet another) bug.
If you're certain the .exe installers for v6.15 are the correct version and haven't been pulled you might want to read 00829955's post yesterday in PLCMAN's Installation Error: V6.09.10300 -- 0X6 This handle is invalid. I recall you posted a workaround for this error that seemed to work for other users and you might be able to help user 00829955.
I also found a few more web pages on the CCleaner site like https://www.ccleaner.com/ccleaner/download where the release notes show the old CCleaner v6.14.10584 (rel. 19-Jul-2023) is the latest available version (even though the "Free Download" button on that page offers the ccesetup615.exe installer), so I sent a support request to support@ccleaner.com on 18-Aug-2023 to ask for clarification about the correct version number for the Portable build and the outdated release notes on many of their web pages. As usual, I didn't get an auto-reply with a ticket number that I can reference, and based on past experience they won't bother to reply to my request for support anyway. As far as I can tell the Avast / Piriform employees have stopped providing support in this forum as well, which doesn't bode well for the future of CCleaner.