Can’t assist much on the first two errors but on the last do you have you email SMTP settings entered correctly in /conf/seahub_settings.py ? Some providers will make you use a third party password instead of your normal password. (For example Apple’s iCloud you must use a third party app password for it to work.)
Can the system send an email for example when you share a library, or folder or etc or do you receive an error for that as well?