Great client, but I can't respond to emails

I really enjoy using MailDroid (2.22 on Kindle Fire). I can access and compose new emails without issues, however when I hit "reply", the screen blackout after a few seconds and display : "Process was killed in background. The message viewer will be closed".

I am down to one account (the only I can't get other mail client to work to send out mail) and it still happening.

Any help or hint ?

Thanks