I received this message, just a few seconds after I began to upgrade my library:
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Upgrading the Zotero database failed:

[Exception... "Component returned failure code 0x80004005 (NS_ERROR_FAILURE) [mozIStorageConection.executeSimpleSQL]" nsresult: "0x80004005 (NS_ERROR_FAILURE)" location: "JS frame :: chrome://zotero/content/xpcom/db.js :: anonymous :: line 410" data: no] [ERROR: constraint failed]
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Now I cannot run Zotero nor access my library. Firefox seems to be working fine though. I am using Mac OS 10.5.6

Thanks for your help