User Agent: Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:60.0) Gecko/20100101 Firefox/60.0
Steps to reproduce:
Few days ago, the certificate used to signing add-ons was outdated and caused all add-ons disabled. I pushed the sync button, trying to fix it. After that, I found the blog post (https://blog.mozilla.org/addons/2019/05/04/update-regarding-add-ons-in-firefox/) and fixed the problem with “hotfix-update-xpi-signing-intermediate-bug-1548973” study. But only few of my add-on (7/4x) remained in the installed list, others were gone. I installed them back and try to sync them again.
I was not able to sync add-ons and got error logs in about:sync-log (see attachment)
Sync add-ons without error and other devices can sync and install new add-ons accordingly.