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.
You can view the errors via Help -> Report Errors to Zotero (at least on OS X—it might be in a different menu on other platforms). Note that errors are cleared when you restart Zotero.
(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?
I am repeatedly getting the message about tags, and am repeatedly asked to resolve differences between the local and remote data -- for the same records. This has now occurred 3x from my office computer. I use Zotero also from 2 other computers.
Thanks for the pointer, yes, my sync is failing with a "!" to the left of the sync button (Standalone Mac). I'm getting a spinning beachball as the sync concludes with the error flag, and Zotero is not responding. So, I cannot get to an error message.
Then, after a another minute or so, Zotero starts the process over again trying to sync.
I'd try disabling auto-sync (Sync tab of preferences), trigger a sync manually (round green arrow), waiting out the beachball, and click on the exclamation mark once Zotero is responding again. That should get you an error message and potentially also a better idea of what's not working.
Today I had the same problem many times with standalone and Firefox:
"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)"
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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.