Wow, thanks a lot!!
There had benn a username in Preferencex.xml (PlexOnlineMail).
I cleared the field, everything is working fine now.
XML
<?xml version="1.0" encoding="utf-8"?>
<Preferences MachineIdentifier="XXX"
ProcessedMachineIdentifier="XXX"
AcceptedEULA="1"
collectUsageData="0"
FSEventLibraryUpdatesEnabled="1"
ScheduledLibraryUpdateInterval="21600"
ScheduledLibraryUpdatesEnabled="1"
autoScanMusicSections="1"
ApertureSharingEnabled="0"
iPhotoSharingEnabled="0"
iTunesSharingEnabled="0"
ButlerTaskGenerateMediaIndexFiles="1"
ButlerTaskRefreshLibraries="1"
CinemaTrailersType="0"
FSEventLibraryPartialScanEnabled="1"
GenerateIndexFilesDuringAnalysis="1"
PlexOnlineMail="demo"
GdmEnabled="1"
allowedNetworks=""/>
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