However, it seems to be related to recurring events:
D/Wire (13260): http-outgoing-0 >> "BEGIN:VEVENT[\r][\n]"
D/Wire (13260): http-outgoing-0 >> "DTSTAMP:20150624T101554Z[\r][\n]"
D/Wire (13260): http-outgoing-0 >> "UID:20150408T070427Z-27766@92a0e4fff7a79fb6[\r][\n]"
D/Wire (13260): http-outgoing-0 >> "DTSTART;TZID=Europe/Berlin:20150408T160000[\r][\n]"
D/Wire (13260): http-outgoing-0 >> "DURATION:PT10800S[\r][\n]"
D/Wire (13260): http-outgoing-0 >> "RRULE:FREQ=WEEKLY;WKST=MO;UNTIL=20150624T135959Z;BYDAY=WE[\r][\n]"
D/Wire (13260): http-outgoing-0 >> "SUMMARY:After-Work-Radeln [\r][\n]"
D/Wire (13260): http-outgoing-0 >> "STATUS:CONFIRMED[\r][\n]"
D/Wire (13260): http-outgoing-0 >> "ORGANIZER:mailto:username@posteo.de[\r][\n]"
D/Wire (13260): http-outgoing-0 >> "LAST-MODIFIED:20150624T101554Z[\r][\n]"
D/Wire (13260): http-outgoing-0 >> "END:VEVENT[\r][\n]"
D/Wire (13260): http-outgoing-0 >> "BEGIN:VEVENT[\r][\n]"
D/Wire (13260): http-outgoing-0 >> "DTSTAMP:20150624T101554Z[\r][\n]"
D/Wire (13260): http-outgoing-0 >> "UID:20150413T095838Z-482@92a0e4fff7a79fb6[\r][\n]"
D/Wire (13260): http-outgoing-0 >> "RECURRENCE-ID:20150415T140000Z[\r][\n]"
D/Wire (13260): http-outgoing-0 >> "DTSTART;TZID=Europe/Berlin:20150415T160000[\r][\n]"
D/Wire (13260): http-outgoing-0 >> "DTEND;TZID=Europe/Berlin:20150415T190000[\r][\n]"
D/Wire (13260): http-outgoing-0 >> "SUMMARY:After-Work-Radeln [\r][\n]"
D/Wire (13260): http-outgoing-0 >> "STATUS:CANCELLED[\r][\n]"
D/Wire (13260): http-outgoing-0 >> "LAST-MODIFIED:20150624T101554Z[\r][\n]"
D/Wire (13260): http-outgoing-0 >> "UID:20150408T070427Z-27766@92a0e4fff7a79fb6[\r][\n]"
D/Wire (13260): http-outgoing-0 >> "END:VEVENT[\r][\n]"
D/Wire (13260): http-outgoing-0 >> "BEGIN:VEVENT[\r][\n]"
D/Wire (13260): http-outgoing-0 >> "DTSTAMP:20150624T101554Z[\r][\n]"
D/Wire (13260): http-outgoing-0 >> "RECURRENCE-ID:20150520T140000Z[\r][\n]"
D/Wire (13260): http-outgoing-0 >> "DTSTART;TZID=Europe/Berlin:20150520T160000[\r][\n]"
D/Wire (13260): http-outgoing-0 >> "DTEND;TZID=Europe/Berlin:20150520T190000[\r][\n]"
D/Wire (13260): http-outgoing-0 >> "SUMMARY:After-Work-Radeln [\r][\n]"
D/Wire (13260): http-outgoing-0 >> "STATUS:CANCELLED[\r][\n]"
D/Wire (13260): http-outgoing-0 >> "LAST-MODIFIED:20150624T101554Z[\r][\n]"
D/Wire (13260): http-outgoing-0 >> "UID:20150408T070427Z-27766@92a0e4fff7a79fb6[\r][\n]"
D/Wire (13260): http-outgoing-0 >> "END:VEVENT[\r][\n]"
Can you please verify that this only happens for recurring events? In this case, it would be a duplicate of #523.