Error #0235: not verified https certificate for host "XXX"
It turned out that the certificate is invalid when downloading or uploading files on MyChat WEB server over https.
This problem can occur when you enable HTTP traffic in MyChat Server settings while using a self-signed certificate. It is not trusted because a browser can't check its validness.
For example, the Android system does not allow downloading/uploading files from WEB servers with untrusted certificates due to security measures. That is why MyChat for Android won't work with invalid certificates.