can you provide a link to the support topic on the github site?

When you go to the download site with a browser. Click this link https://addons.thunderbird.net/en-us/thunderbird/addon/send-later-3/ Click the download button. This should save a file "send_later-10.7.2-tb.xpi" to your downloads folders. In Thunderbird go to the addons and themes tab Click the gear icon on the top tight of the tab. Select install addon from a file from the menu.

Does this issue still occur? I ask, because the download software makes a request for the latest XPI file for the addon. First we need to check that is what is being offered, ie send_later-10.7.2-tb.xpi. Essentially this process is the same as using the automatic download and install, but I have found just sometime is works differently.

If you still had the error Open the error console in Thunderbird (ctrl+Shift+J) use the trash can on it's toolbar to clear everything from the console. try the install again. What appeared in the error console for this action? I would assume there is something about the file/ addon being incompatible.

Hi Matt, many thanks for your help.

I tried the whole gamut you suggested, to no avail. The version of Send Later is the latest 10.7.2

The console indicates: 1759320537991 addons.update-checker WARN Request failed: https://versioncheck.addons.thunderbird.net/update/VersionCheck.php?reqVersion=2&id=sendlater3@kamens.us&version=10.7.2&maxAppVersion=142.*&status=userEnabled,incompatible&appID={3550f703-e582-4d05-9a08-453d09bdfdc6}&appVersion=143.0.1&appOS=WINNT&appABI=x86_64-msvc&locale=en-GB&currentAppVersion=143.0.1&updateType=49&compatMode=normal - [Exception... "Certificate issuer is not built-in." nsresult: "0x80004004 (NS_ERROR_ABORT)" location: "JS frame :: resource://gre/modules/CertUtils.sys.mjs :: checkCert :: line 166" data: no]

The github link as follows: https://github.com/Extended-Thunder/send-later/issues/760#issuecomment-3341405358

Thanks