Error logs about announcementcenter app: error:0406D06E:rsa and error:0906D06C:PEM


I installed announcementcenter and I posted an announcement. However, none user received the email about the announcement (it is enabled in settings).

In the logs (from the page) I am getting:

[notifications] Fatal: error:0406D06E:rsa routines:RSA_padding_add_PKCS1_type_2:data too large for key size at 2019-02-18T08:30:04 +00:00
[notifications] Fatal: error:0906D06C:PEM routines:PEM_read_bio:no start line at 2019-02-8T08:30:04 +00:00


### Server configuration
Nextcloud version : 15.0.4
Operating system and version: Ubuntu 16.04
Apache : Apache/2.4.18 (Ubuntu)
PHP version : PHP 7.0.32-0ubuntu0.16.04.1 => No errors have been found.