Sorry, I didn't explain what happened. I was not able to open any of my files in Zotero. Zotero kept showing the following message: "an error occurred with Zotero".
I am using Iceweasel/Debian.
I restarted the browser two times and then Zotero started to function properly again.
For good measure, check your database integrity from the Advanced → Files and Folders pane of the Zotero preferences. Assuming that passes, don't worry about it, and let us know if it continues to happen after you upgrade to the upcoming Zotero 5.0.
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I am using Iceweasel/Debian.
I restarted the browser two times and then Zotero started to function properly again.
Thank you.