Solved!
While searching in the database for deviating values in table Meter_Calendar, I saw a date value with just a 'yyyy' in stead of 'yyyy-mm-dd'
After updating the record, it was all OK.

Of course the question came up like 'How???'
no clue so far!

Post can be closed!

Hi All, First post here and I'm using Domoticz on my pi2 for over a year with loads of joy and most of the time without any issues. However, since a couple of days, my Gas meter is acting strange. It will take over 20 seconds to show the graphics and if I want to compare the values with last year ...