This will often happen if you leave your machine unused for long periods of time, especially if it has no function to purge the system of water before storage as the Melitta Varianza BTC machine does. Also by neglecting proper mechanical cleaning, not just tossing in a cleaning tab can have the brew unit and surrounding areas full of mould.

Based on the slightly limited information provided, that's all I got for now.

 

Hi there. We were away for one week on holiday but otherwise the machine is used 3-5 times per week. I haven't used cleaning tabs at all and did an internal dry clean a few months ago. Plus descaling. Is there a way to look internally to see if it's full of mould? Thanks.

 

You really are determined to tell me feck all about the machine so I have no idea even how old it is? Get a screwdriver and a torch and have a look inside. If it's security screws, you may have to get the proper bits. You want to look at where the grinds go from the grinder into the brew group and surrounding area..