Login to ucs APPs failed after auto renew certificates letsencrypt

After new year it was not possible to login to any apps that requested LDAP authentification.
Running letsencrypt again does not change anything. System diagnose mentioned:
Ungültiges Zertifikat ‘/etc/univention/letsencrypt/signed_chain.crt’ gefunden:
error /etc/univention/letsencrypt/signed_chain.crt: verification failed

Ungültiges Zertifikat ‘/etc/univention/letsencrypt/signed_chain.crt’ gefunden:
error /etc/univention/letsencrypt/signed_chain.crt: verification failed

After doing Renewing the SSL certificates
and restarting the server:
System diagnose shows the same but it is possible to login into all apps again.
Why do I get the failure after letsencrypt but not when I do renewing the certificates manually?

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