What you had locally is irrelevant — the import is from your online Mendeley library, which includes no information on local file links. For files to be importable, they need to have been uploaded to the Mendeley servers, such that you can open them from your online Mendeley library.
(Zotero tracks file paths and makes them available via syncing in a much more reliable way. It's quite common for new Mendeley installations to lose links to files for this same reason.)
A Mendeley Desktop 1.18 database? We'd probably need to see a copy of the .sqlite file to tell you more. If you don't mind sharing it, you can ZIP it and send it (or a link to it) to support@zotero.org with a link to this thread.
But it'd be better to debug the problems you were having with the online importer. The local option isn't even shown in the import dialog anymore.
Recall I had a problem with a corrupt pdf that cause pdfinfo to hang. After fixing that file, I decided to import the local Mendeley 1.18 database. In Zotero version 5.0.97-beta.33+fdcd4e51c I can import a local Mendeley database by going to File > Import > A file > and selecting the sqlite Mendeley database.
I deleted all Mendeley traces (good riddance), but I can go at it again tomorrow.
Upgrade Storage
(Zotero tracks file paths and makes them available via syncing in a much more reliable way. It's quite common for new Mendeley installations to lose links to files for this same reason.)
A Mendeley Desktop 1.18 database? We'd probably need to see a copy of the .sqlite file to tell you more. If you don't mind sharing it, you can ZIP it and send it (or a link to it) to support@zotero.org with a link to this thread.
But it'd be better to debug the problems you were having with the online importer. The local option isn't even shown in the import dialog anymore.
I deleted all Mendeley traces (good riddance), but I can go at it again tomorrow.