Question is, what formats can you export contacts and tasks?
Going through Thunderbird is a good idea, you couldn’t import at all or did the export to Nextcloud fail?
For the contacts, it seems that you can export to vcards (*.vcf). Perhaps an old format that NC does not support any more but other clients or you can convert to a newer format. I had the problem recently and luckily I found a small script to update the version: Import vcard 2.1 to address book