It works
At the end I changed the selecitzeinput statement in the ui.r
selectizeInput('e2', 'Auswahl', selected = names(db2), choices = names(db2), multiple = TRUE),
and the last statement in my server.r
db2out <- db2out[db2out$Metric.ID == input$MetIDlist,] # Missing comma at the end
db2out[, input$e2, drop = FALSE]