The total traffic is rounded to 2 decimal places in PerformanceReport.java by converting it to a string and back.
That fails with a system locale using a comma to separate the decimal places.

Exception in Jenkins log:

Caught exception evaluating: it.getTotalTrafficInKb() in ... Reason: java.lang.NumberFormatException: For input string: "5351,75"
java.lang.NumberFormatException: For input string: "5351,75"
at sun.misc.FloatingDecimal.readJavaFormatString(FloatingDecimal.java:1250)
at java.lang.Double.valueOf(Double.java:504)
at hudson.plugins.performance.PerformanceReport.roundTwoDecimals(PerformanceReport.java:325)
at hudson.plugins.performance.PerformanceReport.getTotalTrafficInKb(PerformanceReport.java:211)