xcube serve can not create time series for some datasets

[TonioF]

For some datasets, requesting a timeseries will result in a ValueError. We observed this for datasets on our development DOORS xcube-viewer (from partners METU and ISMAR-CNR). If you set a geometry and request a time series for it, you will be given a non-telling error message. Browsing through the xcube-server logs, we found stack traces posted below.

Our initial assumption was that the time values were unsorted or non-unique, but this wasn't the case. We now assume that the problem occurs because the datasets in question hold four-dimensional variables (ordered time, xxx, lat, lon).
stacktrace1.txt
stacktrace2.txt

[forman]

Related: xcube-dev/xcube-viewer#239