Opening mailbox file using PHP gives "Can't open mailbox" error

Hello,

I'm using this the c-client library via PHP.
While most of the time it works, sometimes i get an error message 
stating "Can't open mailbox emailmessage : no such mailbox (errflg=2)".

I can confirm that the file really exists, is readable, not locked or 
whatever.
Also a second attempt (or third etc..) on the same file during the same 
session doesn't work.
The weird thing is that it sometimes works, and sometimes not, even 
while the file/content is exactly the same.

What am i doing wrong?
Is this some known bug?
Or how can i work around this?

Thanks in advance.

Mathijs.

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