In SalesLogix 7.5.1 Web client on a SQL2005 database there is a known defect where null values in the activity table prevent users from seeing activities in the Activities View.  We have used scripts to correct the issue but those scripts must be run every time the issue reappears.  Here is a trigger that we are using to correct the null values.

Create Trigger UpdateNulls
on sysdba.Activity
for insert
as
 
If (select count (*) from Activity where recurring is null) > 0
begin
 
update sysdba.Activity set recurring = 'F' where recurring is null
end
 
If (select count (*) from Activity where rollover is null) > 0
begin
 
update sysdba.Activity set rollover = 'F' where rollover is null
end
 
If (select count (*) from Activity where timeless is null) > 0
begin
 
update sysdba.Activity set timeless = 'F' where timeless is null
end
 
go

If you are experiencing this issue run the following scripts in the Administrator against your SalesLogix database and then add the trigger.

update activity set recurring = 'F' where recurring is null
update activity set rollover = 'F' where rollover is null
update activity set timeless = 'F' where timeless is null

The trigger should keep the null issues from reappearing.  As a precaution create a SQL backup prior to running any scripts are adding any triggers.  Good Luck!