Our current release versions have the option in Sync to also synchronize add-ons.
The way this works is that a list of the add-ons gets synchronized to other devices, and then the target device (if of the same type, e.g. desktop) will launch a search for the list of add-ons you have, and downloads them directly.
Many of these searches, however, will be forwarded to addons.mozilla.org, that doesn't provide a proper response to these searches for add-ons and will send browsers the latest version of the add-on they have listed, regardless of the version that was installed on your other device! We're assuming at this point that this (undocumented) type of search query no longer caters to requests in the way we send it out - changed possibly when Mozilla transitioned to Firefox Accounts instead.
Getting these add-ons and having the sync client install them will end up with incompatible add-ons in your list.
For the time being, we're stopping these requests from being sent to AMO on the server side, and Pale Moon 27 will no longer offer the option to sync add-ons because it relies indirectly on AMO.
A kind request to everyone who has this option enabled to go into Options, Sync, and disable it -- for your own protection of getting incompatible add-ons sent to you as well as to reduce unnecessary requests to our servers (both sync and addons server).
Thanks in advance!
Anything to do with the Pale Moon Sync service.