First, you're getting a bunch of errors downloading files with filenames containing extended characters. This has already been fixed, and the fix will be included in the next release.
For the main error you're getting, see the filename in the error dialog. Rename that file on the other computer, relink it in Zotero, and sync that computer again. Then sync this side.
Would it help to use the Development XPI? I did not experience this problem until I reinstalled my secondary computer? I have a lot of filenames with umlauts and I don't want to change them
Don't use the dev XPI at the moment. The next release candidate build will be out today and should fix the umlaut issue.
For the main error, take a look at the file in question, which has an apostrophe in its filename.
1) What OS are you syncing from? Does the apostrophe appear correctly in the filename (when you look via the OS) there?
2) How did you create the parent item and attachment? Did you create both via a translator? Did you retrieve the parent item from somewhere (where?) and use Rename File from Parent Metadata? Did you download the attachment directly without renaming it?
Upgrade Storage
For the main error you're getting, see the filename in the error dialog. Rename that file on the other computer, relink it in Zotero, and sync that computer again. Then sync this side.
johan
For the main error, take a look at the file in question, which has an apostrophe in its filename.
1) What OS are you syncing from? Does the apostrophe appear correctly in the filename (when you look via the OS) there?
2) How did you create the parent item and attachment? Did you create both via a translator? Did you retrieve the parent item from somewhere (where?) and use Rename File from Parent Metadata? Did you download the attachment directly without renaming it?