Inccorect Hreflang

Since i changed to 4.4.5 i lost half the traffic.I was wondering what happend and today i received this mesage in Google Search Console
(note: i use 2 languages for the site)

"Google has detected that some pages on your site have implemented the rel-alternate-hreflang tag incorrectly. In particular, there seems to be a problem with incorrect language and region codes, or incorrect bi-directional linking (if page A links with hreflang to page B, there must be a link back from B to A as well). Google uses the hreflang attributes to serve the correct language or regional URL to the right users in search results.
Until you correct the errors on the hreflang links, your website will not benefit from additional language or region targeting."

If i browse the example they showed me the error is " no return tags"

What exactly is the problem?

Have the same probs with hreflang and the sitemap for Google has some issues too. I lost a lot of Websites from Google index :(

Any update on this? Got a same problem...