Same thing happend to me while ago it turned the calendar folder had been used to install scripts by a hacker.
they had left a file in the calendar folder called updates/pro.pl
I Didnt have the latest update 3.21 installed. I HAVE NOW.
According to my Host This is what the file could do.
"""this program allows the hacker to enter any
shell command they want via a web browser and run it on your account.
This is normally used to download and install other code into your web
space."""
Latest update is 3.21 i would suggest instaling this. and having a Look round your calendar-cgi-bin for any extra files folders.
Let me know if it helped you
bye.
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Nel