Unsupported operand types: string + string at /usr/share/davical/inc/caldav-ACL.php:147

We're seeing the following error message in the error.log of one of our servers:

host ~ # cat /var/log/iserv/server-davical/error.log
[22-Mar-2023 08:37:18 Europe/Berlin] PHP Early Exception: [0] Unsupported operand types: string + string at /usr/share/davical/inc/caldav-ACL.php:147
[22-Mar-2023 08:37:28 Europe/Berlin] PHP Early Exception: [0] Unsupported operand types: string + string at /usr/share/davical/inc/caldav-ACL.php:147
[22-Mar-2023 10:34:40 Europe/Berlin] PHP Early Exception: [0] Unsupported operand types: string + string at /usr/share/davical/inc/caldav-ACL.php:147
[22-Mar-2023 10:35:40 Europe/Berlin] PHP Early Exception: [0] Unsupported operand types: string + string at /usr/share/davical/inc/caldav-ACL.php:147
[22-Mar-2023 10:37:48 Europe/Berlin] PHP Early Exception: [0] Unsupported operand types: string + string at /usr/share/davical/inc/caldav-ACL.php:147
[22-Mar-2023 10:37:52 Europe/Berlin] PHP Early Exception: [0] Unsupported operand types: string + string at /usr/share/davical/inc/caldav-ACL.php:147
[22-Mar-2023 14:57:45 Europe/Berlin] PHP Early Exception: [0] Unsupported operand types: string + string at /usr/share/davical/inc/caldav-ACL.php:147
[22-Mar-2023 14:58:42 Europe/Berlin] PHP Early Exception: [0] Unsupported operand types: string + string at /usr/share/davical/inc/caldav-ACL.php:147
[22-Mar-2023 14:59:03 Europe/Berlin] PHP Early Exception: [0] Unsupported operand types: string + string at /usr/share/davical/inc/caldav-ACL.php:147
[22-Mar-2023 14:59:19 Europe/Berlin] PHP Early Exception: [0] Unsupported operand types: string + string at /usr/share/davical/inc/caldav-ACL.php:147
[22-Mar-2023 14:59:43 Europe/Berlin] PHP Early Exception: [0] Unsupported operand types: string + string at /usr/share/davical/inc/caldav-ACL.php:147
[22-Mar-2023 15:05:45 Europe/Berlin] PHP Early Exception: [0] Unsupported operand types: string + string at /usr/share/davical/inc/caldav-ACL.php:147
[22-Mar-2023 15:10:02 Europe/Berlin] PHP Early Exception: [0] Unsupported operand types: string + string at /usr/share/davical/inc/caldav-ACL.php:147
[22-Mar-2023 15:14:03 Europe/Berlin] PHP Early Exception: [0] Unsupported operand types: string + string at /usr/share/davical/inc/caldav-ACL.php:147
[22-Mar-2023 15:14:38 Europe/Berlin] PHP Early Exception: [0] Unsupported operand types: string + string at /usr/share/davical/inc/caldav-ACL.php:147
[22-Mar-2023 15:16:51 Europe/Berlin] PHP Early Exception: [0] Unsupported operand types: string + string at /usr/share/davical/inc/caldav-ACL.php:147
[22-Mar-2023 15:16:53 Europe/Berlin] PHP Early Exception: [0] Unsupported operand types: string + string at /usr/share/davical/inc/caldav-ACL.php:147

This looks like a simple mistake in the code: https://gitlab.com/davical-project/davical/-/blob/master/inc/caldav-ACL.php#L146

The strings should be concatenated with ., not with +.