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Dan would have to say what can cause the "tag merge" message to come up - I think it's not that uncommon.
As a general note, it is not recommended to use sync instead of a regular back-up. While it will, in most cases, be able to restore your database, you should always have an automatic local back-up (for Zotero as for all of your important files and documents) as the go-to solution to restore your data.
Don't know about the error console from standalone - it's beta software, so not all messages have been fixed and some still refer to the FF version.
(The Error Console currently shows in Standalone only if you start it from the command line with the -jsconsole flag.)
Looking at your sync history, I see mostly uploads, as expected, and only four small downloads going back to June. One of those was a couple hours ago, which is presumably what triggered that message. Are you sure you didn't make changes on zotero.org?
Report ID 1842599343
"Uno o más Zotero tags localmente eliminados han sido modificados remotamente desde la última sincronización. Se han conservado las versiones remotas.
Ver la consola de error Firefox para la lista completa de tales cambios."
(Free translation: One or more Zotero tags locally elliminated has been remotely modified since last sincronization. Remote versions has been conserved. Look Firefox error console for complete list of changes).
I had disable automatic sync in both versions, run a manual sync, but problem remains. Restart Zotero, Firefox and computer, without success.
When I click on red "!" this message appears: "Uploaded data not well-formed (Report ID: b70ad4d972)"
Error ID 647063903
Thanks if you can help.