Dear Peter,
Thank you for your response.
I have resolved the issue by resetting the mailbox size of that particular account to default, then adjust the mailbox size again. I haven't test with other account yet, will get back to you once I have verified if such could resolve other accounts as...
Is there anyway not letting roundcube webmail showing default mailbox size but just show the assigned mailbox size, as our client had assigned different mailbox size to different user.
Many thanks in advance for advice.