exchangelib: Cannot convert _start_timezone

Tweaking your example code, I fetch the calendar entries, using this:

...
    ids=account.calendar.filter(start__range=(
        EWSDateTime.from_datetime(dt_start),
        EWSDateTime.from_datetime(dt_end)
    )).only('subject', 'start') 
...

I get warnings like this

WARNING:exchangelib.fields:Cannot convert value 'Europe/Copenhagen' on field '_start_timezone' to type <class 'exchangelib.ewsdatetime.EWSTimeZone'> (unknown timezone ID)
WARNING:exchangelib.fields:Cannot convert value 'Europe/Paris' on field '_start_timezone' to type <class 'exchangelib.ewsdatetime.EWSTimeZone'> (unknown timezone ID)

and the resulting start time for my calendar entries are off by 2 hours.

I have no idea where to start debugging this. Could I be missing some packages?

Some info: INFO:exchangelib.version:API version “Exchange2013_SP1” worked but server reports version “V2_23”. Using “Exchange2013_SP1”

➜ python --version Python 2.7.13

➜ pip list | grep exchange exchangelib (1.10.4)