Thanks. The stack trace points out it's the alarm element.
SOAP API for 4.5 said alarm element had a single attribute called rel-start, but it actually wasn't implemented and was inadequate to describe a RFC2445 VALARM component. In 5.0 alarm element was redefined. The relevant section in soap-calendar.txt is:
// VALARMs (RFC2445 Section 4.6.6)
[
<alarm action="DISPLAY">
<trigger>
// <rel> has the same attributes as <dur> and an optional
// "related" attribute. Default value of "related" is "START".
<rel [related="START|END"] .../> OR <abs d="YYYYMMDDThhmmssZ"/>
</trigger>
<desc>{reminder text to display}</desc>
// <repeat> has the same attributes as <dur> and an additional
// required count attribute. The duration is how often to repeat
// the alarm, and the count is how many times to trigger the
// alarm IN ADDITION TO the initial alarm.
[<repeat count="N" .../>]
</alarm>
]*
[
<alarm action="AUDIO">
<trigger/> // same as in DISPLAY alarm
[<repeat/>] // same as in DISPLAY alarm
[
<attach ct="{content type}" uri="{uri}"/>
OR
<attach>{base64-encoded binary data}</attach>
]
</alarm>
]*
[
<alarm action="EMAIL">
<trigger/> // same as in DISPLAY alarm
[<repeat/>] // same as in DISPLAY alarm
<desc>{email body}</desc>
<summary>{email subject}</summary>
<at .../>+ // attendees (one or more email recipients)
[<attach ... />] // sam as in AUDIO alarm
</alarm>
]*
[
<alarm action="PROCEDURE">
<trigger/> // same as in DISPLAY alarm
[<repeat/>] // same as in DISPLAY alarm
[<desc>{description text}</desc>]
<attach ... /> // same as in AUDIO alarm
</alarm>
]*
The syntax is pretty much a one-to-one mapping from RFC2445 VALARM.
Most alarms will have DISPLAY action value. The other action types are there simply because the RFC defines them. Each client must deal with the types as appropriate. For a mobile client, I would expect a pop up alert of some sort is adequate in all cases.