Disappearance of network sites, after upgrade to 4.7.3

I have updated a WordPress network site to version 4.7.3. Sadly, that has unleashed some sort of disaster. All network sites have disappeared after this, as well as the network admin interface. Only the main (mother) blog is still accessible. All network users are still there.

When I take a look at the wp-config.php file, it shows that the network (i.e. multisite) definitions have been erased.

Taking a look at the corresponding database, shows that all network sites are somehow still there. However, directly trying to open the link of such a site results in an error page.

I figured that perhaps adding multisite definitions back to the wp-config file might solve the problem. Not really, since the effect is that on the mother blog a warning disappears. Namely that a network for this network site already exists and network re-installation should not be done.

So the network is virtually still there, but the upgrade made it inaccessible.

Would really appreciate some help -on short notice- here!