Reported by potential focus customer and reproduced internally.

Tried to install Nextcloud on member server.
Symptom: Pending join script (50nextcloud).
Even after running univention-run-join-scripts the join script doesn't run successfully.

/var/log/univention/join.log:
During Join:
Updating certificates in /etc/ssl/certs... 0 added, 0 removed; done.
Running hooks in /etc/ca-certificates/update.d....done.
....
During Installation of Nextcloud:
++ curl -X POST -H 'OCS-APIREQUEST: true' -u 'nc_admin:iu(g3Que=ra7dae-kailee4uGh-esh' https://nextcloud.schulen.XY.univentiontest/nextcloud/ocs/v2.php/apps/user_ldap/api/v1/config
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current Dload  Upload   Total   Spent    Left  Speed
^M  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0curl: (60) SSL certificate problem: self signed certificate in certificate chain
More details here: http://curl.haxx.se/docs/sslcerts.html

curl performs SSL certificate verification by default, using a "bundle"
 of Certificate Authority (CA) public keys (CA certs). If the default
 bundle file isn't adequate, you can specify an alternate file
 using the --cacert option.
If this HTTPS server uses a certificate signed by a CA represented in
 the bundle, the certificate verification probably failed due to a
 problem with the certificate (it might be expired, or the name might
 not match the domain name in the URL).
If you'd like to turn off curl's verification of the certificate, use
 the -k (or --insecure) option.
+ RESULT=
++ grep '<statuscode>200</statuscode>' -c
++ echo
+ STATUS=0
+ '[' '!' 0 -eq 1 ']'
+ die 'Could not create LDAP Config at Nextcloud'
+ exit 0
EXITCODE=0

All works after running:
root@nextcloud:~# update-ca-certificates --fresh
Clearing symlinks in /etc/ssl/certs...done.
Updating certificates in /etc/ssl/certs... 174 added, 0 removed; done.
Running hooks in /etc/ca-certificates/update.d....done.

To me this are two failures:
1. The join scripts returns EXITCODE=0 even after it fails.
2. The univention-join doesn't run "update-ca-certificates" properly.