Array and string offset access syntax with curly braces is deprecated at /var/www/nextcloud/apps/rainloop/app/rainloop/v/1.14.0/app/libraries/MailSo/Mime/Email.php#105

Server: ubuntu 18.04.4
DB: PostgreSQL 12.2
PHP: 7.4.5

First time i recognizes this error was under PHP 7.4.5 with Nextcloud Server 18.0.4.2
and Rainloop Modul Version 6.1.0 installed

My Log shows the following errors

[PHP] Error: Array and string offset access syntax with curly braces is deprecated at /var/www/nextcloud/apps/rainloop/app/rainloop/v/1.14.0/app/libraries/MailSo/Mime/Email.php#105

POST /apps/rainloop/app/?/Ajax/&q[]=/_fQukx7WW_QguG6SFzoYxxV_vMpegfUphrKKPWzIJxY8gA0bWTxfU3ZR2w-1ewT9hyHHAAansoORlprCTbgquD-aNs7uoG1pNJCXe3ckVsX8BHD16G0P8868nJv-8SIi75sZYptGLrlxkU5oX-pdU0lEjL1sHtL32ueD8K0ezXxDKKnN3mMoLHg6pAGkA2W1osw7tI62s_fVYuWc1KXHMDcBajCt9Q2LPIgrJiCbG5YWqHo62Kv-a_UqSX3FL4j_4scbtQeRnHvCLpTvXW3AO9iNeE1SWwDIhVV5RsmgneWi4xXkHtZ-OXWACL5T737l_vYdJJ5f8I-RUcpM96PKhjVEEpyCsL8ZnJ8zkFA/
from 192.168.55.121 by christian at 2020-05-08T09:22:40+02:00

[PHP] Error: Array and string offset access syntax with curly braces is deprecated at /var/www/nextcloud/apps/rainloop/app/rainloop/v/1.14.0/app/libraries/MailSo/Base/Utils.php#2143

GET /apps/rainloop/app/?OwnCloudAuth
from 192.168.55.121 by oca at 2020-05-08T12:00:21+02:100:

and

[PHP] Error: Array and string offset access syntax with curly braces is deprecated at /var/www/nextcloud/apps/rainloop/app/rainloop/v/1.14.0/app/libraries/lessphp/lessc.inc.php#3071

GET /apps/rainloop/app/?/Css/0/User/-/Default/-/0f02fb6f9897aaf90b9e885d94f38646/Hash/-/
from 192.168.55.121 by oca at 2020-05-08T12:00:21+02:00

As far as i could test it, my rainloop app seems to work with getting / sending emails.
Not sure if it has to do with the PHP version 7.4 instead of 7.3.x
With the 7.3 i did not get those errors but there where earlier versions of the rainloop app too !

Append:

user: with error = … v/1.14.0/app/libraries/MailSo/Mime/Email.php#105
has made settings for his credentials in the additional settings page of nextcloud

user: with errors = …/v/1.14.0/app/libraries/MailSo/Base/Utils.php#2143
and … v/1.14.0/app/libraries/lessphp/lessc.inc.php#3071
has empty credentials for login to mailserver over rainloop app - so will be stoped at the login prompt.

Maybe this is important for further examinations.