I use Baikal as DAV server and DAVdroid 1.3.1 from F-Droid
and I get an exception DUE must not be < DTSTART every time
full debug info here https://paste.fedoraproject.org/447730/14761190/
I manually checked all my 100+ tasks and there not a single entry with such problem, including completed
all tasks has DUE > START date
to create tasks I use 2Do (macOS) and OpenTasks (Android)
and previously I use CalDAV-Sync, it never complained, but I wanted CalDAV/CardDAV sync in one app, that’s why I switched to DAVdroid
@rajegyb There’s surely a task with DUE < DTSTART, which can’t be synchronized (because it’s invalid and OpenTasks doesn’t accept it). You can see more information when you enable verbose logs.
Please follow up at
@rfc2822 CalDAVSync (not free) is working with that data
I tried to make DAVDroid work with my server, but I can’t find the problematic tasks (even with verbose logs enabled)
So I ended up paying for another app in Google Play…
@rajegyb In future DAVdroid versions, ical4android will ignore invalid DUE < DTSTART values.
Please keep in mind that this is a dirty workaround. If you want to put efforts in this problem, please fix the clients which create invalid tasks.
Seems the DAVdroid now becomes an iCalendar validation and repair tool, too … 
Has been implemented with DAVdroid 1.3.3, so I’m setting this to solved.
