Query was cancelled - reason?

Hello,

i'am currently run into a strange problem with fast subsequent updates in the same table.

The application is quite complex, so its difficult to to sent a code snippet, i included the network traffic instead:

This is whats happening:
---
T 192.168.195.100:5432 -> 192.168.195.1:56810 [AP]
CBEGIN.Z

T 192.168.195.1:56810 -> 192.168.195.100:5432 [AP]
QSELECT * FROM vsysuserfilter_106_u WHERE filter_id=10 FOR UPDATE OF vsysuserfilter_106_u.

T 192.168.195.100:5432 -> 192.168.195.1:56810 [AP]
Pblank.T..datasource..........Dfilter_column...........filter_id...........filter_string...........filter_type...........userremark...........D.....vtblemloyee_firm_groups....eMail....10....mail....1....mailCSELECT.Z

T 192.168.195.1:56810 -> 192.168.195.100:5432 [AP]
QSELECT * FROM vsysuserfilter_108_u WHERE filter_id=10 FOR UPDATE OF vsysuserfilter_108_u.

T 192.168.195.100:5432 -> 192.168.195.1:56810 [AP]
Pblank.T..datasource..........Dfilter_column...........filter_id...........filter_string...........filter_type...........userremark...........D.....vtblemloyee_firm_groups....eMail....10....mail....1....mailCSELECT.Z

T 192.168.195.1:56810 -> 192.168.195.100:5432 [AP]
QUPDATE vsysuserfilter_106_u SET "filter_column" = 'eMail' WHERE "filter_id" = 10.

T 192.168.195.100:5432 -> 192.168.195.1:56810 [AP]
Pblank.CUPDATE 1.Z

T 192.168.195.1:56810 -> 192.168.195.100:5432 [AP]
Qcommit;begin;.

T 192.168.195.100:5432 -> 192.168.195.1:56810 [AP]
CCOMMIT.EERROR: Query was cancelled...
---

So why comes the commit-error?

Notes:
The select-for-updates above go on views which have update-rules, both views go on the same target table and record but do have different columns included, not always both are required, but
that can't be known at the time the select-for-update is initiated. Not sure if that is important here.

I already had a look into the postmaster log at debug level 5 and found nothing suitable.

Thanks for any help,
Guido