I found Count Posadowsky-Wehner right away in an initial Google search, in a Who is Who style work from the 1920s:
Hermann A. L. Degener (ed.), Wer ist's. VIII. Ausgabe. Leipzig: H. A. Ludwig Degener 1922, p. 1784:
Posadowsky-Werner, Torsten Gf. v., Mil. Att., † 7. X. 14 b. Wodzilki a. d. Szeszupa
His stated place of death is readily identified as the small village of Wodziłki, now located in the north-eastern corner of Poland near the small river Szeszupa. The village is located about 40km east of Gołdap, Poland.
There was fighting in this area in the aftermath of the First Battle of the Masurian Lakes, with German troops pushing Russian forces back into Russian territory, roughly between Wirballen (now Virbalis, Lithuania) and Suwalki (now Suwałki, Poland).
I found a book by someone who actively participated in the fighting who, from cursory reading, appears to have been a low-ranking officer:
Friedrich Immanuel, Was ich in mehr als 80 Schlachten und Gefechten erlebte. Berlin: E. S. Mittler und Sohn 1916
I consulted pp. 40-55. Immanuel's unit reached Wirballen on September 20, 1914. On September 27, they advanced to Mariampol (now Marijampolė, Lithuania). Around September 30, they reached Kalwarja (now Kalvarija, Lithuania) with the goal of advancing towards Suwalki. But shortly after leaving Kalwarja they encountered relatively strong Russian cavalry. Fighting, including a tactical retreat back towards Wirballen, continued until the end of the month when the German forces finally entered Suwalki on October 27, 1914.
If one draws a line on the map between Virbalis and Suwałki, one finds that Wodziłki lies almost exactly on this line.
I also consulted a multi-volume work on WW1 published by the German Government Archives (Reichsarchiv (eds.), Der Weltkrieg, 1914 bis 1918, Band 2. Berlin: E. S. Mittler und Sohn, 1925) to corroborate Immanuel's narrative of events, but found that it does not contain details of the fighting in the East Prussian border region after mid-September 1914.
Hermann Stegemann, Geschichte des Krieges, Zweiter Band. Stuttgart and Berlin: Deutsche Verlags-Anstalt 1918, p. 202ff. describes how the Russian general Rennenkampf, after adding fresh forces during the third week of September, led a counter offensive that commenced on September 28, 1914. One branch of it pressured German forces on the line Mariampol - Kalwarja - Suwalki - Augustow (now Augustów, Poland) by October 2, with some local successes by October 9. Heavy fighting continued until October 12, with the relatively weak German forces in the area barely holding on until they regained the initiative by October 15. By the end of October the Russian counter offensive had failed.
