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
dohave 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