Nokia is involved in a series of legal proceedings against Oppo in Germany and other European countries regarding patent infringement. After selling the Devices and service business to Microsoft in 2013, Nokia doubled down on monetising its patent portfolio and had the leverage of not being in the smartphone market, which made counter-suing Nokia hard for smartphone makers.
In 2018, Nokia signed a patent licensing deal with Oppo, but that deal expired last year triggering a series of lawsuits that also lead to ban of sales for Oppo/OnePlus devices in Germany. FossPatents took a closer look at Oppo's and OnePlus' availability in Germany and also mentioned that for Oppo it might make sense to forgo the German market to avoid paying for sales on a global level. Oppo is also under pressure from other patent holders besides Nokia. Furthermore, as Fosspatents mentions, this is the first time a major phone maker is completely exiting the German market. I also recommend checking the full article there as it contains interesting information and analysis.
Editor: IPR Daily-Selly