Same answer from the other report. We need a Debug ID using "Restart with Logging Enabled". If you can't access the menu, see Reporting Startup Errors.

Can you copy and paste the full output and email it to support@zotero.org with a link to this thread? (We'll respond here.)

(2)(+0000002): Subject not found: http://zotero.org/users/local/R6TvOA40/items/Z834P7NM - dc:replaces - http://zotero.org/users/local/R6TvOA40/items/6V2RWEMI

(4)(+0000003): INSERT INTO relationPredicates (predicateID, predicate) VALUES (NULL, ?) ['dc:replaces']

(4)(+0000012): SELECT predicateID FROM relationPredicates WHERE predicate=? ['dc:replaces']

(1)(+0000063): Rolled back DB transaction M83NgtMF

(1)(+0000005): target is undefined

Weird.

If you're willing to share the DB with us, either by uploading it somewhere and emailing us a link or, if it's not too big, uploading it to the DB Repair Tool and posting the Upload ID, we can try to reproduce this and see what's going on. (If you use the DB Repair Tool, you can ignore the download link the tool provides, which won't help.)

Also, just to be clear, if you're not running the official Zotero build, you should try that first. You can also try holding down Shift at startup to disable third-party plugins. This error shouldn't really be possible.

Update: I tried holding down Shift at startup to disable third-party plugins, but still get the same error.

Is this error related to the
Subject not found: http://zotero.org/users/local/R6TvOA40/items/Z834P7NM - dc:replaces - http://zotero.org/users/local/R6TvOA40/items/6V2RWEMI ? If so, how can I reversely locate the zotero item within Zotero 4 for Firefox?

For privacy concern, I can't share my zotero.sqlite...

I'll try using the official non-portable version of Zotero soon, but before that, here is what I get after opening the zotero.sqlite (after copying from \PortableApps\FirefoxPortableESR\Data\profile\zotero\ to \PortableApps\ZoteroPortable\Data\profile\ and launching Zotero 6 to produce the error):
https://user-images.githubusercontent.com/104580395/165879309-5e476818-cb5a-4baf-ac42-a4aa7b1a6175.png

There seems to have several non-unique item keys such that some key is replaced by the more than one key (or different keys are replaced by a same key). Is this the reason of this error?

OK, I'll try that later, as it will take some time to test removing tables while still being able to produce the error.

As a last resort as you suggest, if I export the entire library in Zotero 4 with Firefox to Zotero RDF, and then import it into Zotero 6, I will "lose the link to those items in word processor documents" according to https://forums.zotero.org/discussion/comment/386622/#Comment_386622. But fortunately, I didn't really use this particular database with MS Word. However, I did use it heavily with Better BibTeX. So what else will I lose other than links to those items in word processor documents? And will that affect Better BibTeX's automatic export of collections?

I've installed the official non-portable version of Zotero, and change the datadir to where the zotero.sqlite is located (U:\PortableApps\ZoteroPortable\Data\profile) in order to open that database, and I still get the same error.

I'm trying to empty the tables of zotero.sqlite containing private data now, but it doesn't allow me to do so: https://user-images.githubusercontent.com/104580395/165884800-7be4a3f8-547d-4ab2-bc5c-403d4e8857d3.png. Is there any method to force deletion?

PRAGMA foreign_keys = OFF might do it. Otherwise you'd have to delete in the order of the foreign key constraints.

Sorry, but we can't help you further without the database.

We have no interest in your data and will delete the database immediately after trying to reproduce this. I completely understand if you still don't want to share it, but then I'm afraid you're on your own for debugging this, replacing/removing private values from the database so you can share a stripped down version, or finding another workaround to upgrade to a current version.

OK, I was able to anonymize my zotero.sqlite by using SQL queries like (after removing the UNIQUE constraints on the columns to be anonymized):
UPDATE collections
SET collectionName = collectionID

These queries set the private and sensitive names/values to the IDs which are integers and unique within each table. And I've also set NULL to some columns where this is allowed.

And I can reproduce the same error message of _migrateUserData_80_relations@chrome://zotero/content/xpcom/schema.js:3660:34, using BOTH the official non-portable version of Zotero and the portable version of Zotero.

Now I've uploaded my zotero.sqlite to https://www.zotero.org/utils/dbfix/, and the Upload ID is 626d37e414e4e

So @dstillman , when can I expect a fix of this issue? If it takes days (which I can't wait for, as I need to use it now or soon, even though I'll have to use it temporarily with Zotero 4 for Firefox and upgrade the database using Zotero 6 later), will it help if I try upgrading to some sub-version of Zotero 5.0 first?

Thanks a lot! I can confirm that the beta version works. But extra steps of switching to a beta version for database upgrade and switching back to the release version is needed, and I've been using the old database these days with Zotero 4 for Firefox and many changes have been made to the old database, so another upgrade will be needed. Therefore, I'm wondering when will the next release version become generally available for download. I hope it will be very soon, and if not, could you please provide an estimated date of release? Many thanks again!