My Juris-M 5.0.52m40 installations throws the following error on each sync:

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[JavaScript Error: "Error 400 for item 6A7YLQHK in My Library:

ZoteroObjectUploadError: '' is not a valid linkMode"]

[JavaScript Error: "'' is not a valid linkMode" {file: "chrome://zotero/content/xpcom/sync/syncEngine.js" line: 1318}]

[JavaScript Error: "Made no progress during upload -- stopping" {file: "chrome://zotero/content/xpcom/sync/syncEngine.js" line: 1377}]

[JavaScript Error: "Made no progress during upload -- stopping" {file: "chrome://zotero/content/xpcom/sync/syncEngine.js" line: 1377}]
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Report ID: 1546951150-17772, 1546951464-63587 .

I found a discussion with a similar error message here: https://forums.zotero.org/discussion/73083/re-multiple-errors-while-syncing-inconsistent-items-in-group-library , and it looks like you might be interested in getting more details about this. (But I'm not using stock Zotero, sorry.)

There's indeed something wrong with the item 6A7YLQHK - I'm even unable to "expand" it. It looks like trying to do so created the following messages in the log:
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[JavaScript Error: "UnloadedDataException: 'childItems' not loaded for item (1903/1/6A7YLQHK)"]
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Bibliography fields look OK though. Double-clicking the item opens an url with pdf (not local pdf file).

I've added this item yesterday. Via Jiris-M's Chrome connector (when viewing this pdf). The URL is https://mipt.ru/education/post-graduate/D212-156-06/16/Advisor_Review_Chernousov.pdf . AFAIU, I've created the "parent item" and manually edited some fields (title, author, date) shortly after grabbing the item.

The "check database integrity" says "Errors were found in your Zotero database. Zotero can attempt to correct these errors. Zotero will need to be restarted.", but clicking "Fix Errors and Restart Zotero" changes nothing - no restart happens, and the issue is still present after a manual restart. (Maybe this button is even non-functional in Juris-M, I'm not sure.)
(Just in case: I've created a full backup of my Zotero folder, so, probably, I would be able to reproduce this.) I even don't see anything new in the log. But just in case, here's one more Report ID: 1546952937-11546 .