What is the origin of the Laacher See caldera magma in Germany?
The East Eifel Volcanics, of which the Laacher See Caldera is a part of, is located on the Rhenish Shield and is associated with the Cenozoic Upper Rhine Graben continental rift zone (Aber, 2012). 

Ukrich and Gabrielle (2013) hypothesise that as the region did have and still has active tectonics, that the magma chamber for the Laacher See Caldera could have been the result of magma accumulating in voids opened up through regional strike-slip faulting (including the 'Laacher See Strike Slip' and Ochtending Fault), block rotation and regional uplift. 

According to Schmitt et al. (2010), the parental magmatic melt that formed the carbonatite-syenite suite based petrology that dominates the Laacher See caldera is derived from a mantle melt differentiated to phonolite.

It is believed that the magma resided/accumulated for around 20,000 years prior to the eruption.  Schmitt et al. hypothesise that the magma had a cooler and older outer edge, compositionally zoning to a hotter phonolitic core - the presence of younger phenocrysts could indicate a magma reorganisation just prior to the eruption.

References

Aber, 2012, Volcanism of the Eifel, Germany Region, GO 326 Plate Tectonics course notes

Schmitt et al. 2010, Magmatic Longevity of Laacher See Volcano (Eifel, Germany) Indicated by U-Th Dating of Intrusive Carbonatites, Journal of Petrology

Ulrich and Gabrielle, 2013, Why does the Size of the Laacher See Magma Chamber and its Caldera Size not go together? - New Findings with regard to Active Tectonics in the East Eifel Volcanic Field, EGU General Assembly 2013