Can't access add/delete in Yahoo Calendar

We have 3 medium-sized LANs and everyone needs to access Yahoo Calendar - and this would be one of the pages:
http://www.calendar.yahoo.com/YYY,498696/srt,0/?v=150&pv=1&i=0&a2=1&t=1262779038
Adding a calendar event will time out for some of the computers.

2 of the 3 sites work fine.  And, at least one of the computers on the 3rd site works fine.  However, it seems that none of the other computers at the 3rd site can effectively use Yahoo Calendar as above.  
This trouble started only very recently.
All of the computers have TrendMicro Internet Security Installed.
All of the computers are behind the same type firewall (or firewall setup at each site).
At the site with the trouble, the computer that works is closer physically to the firewall via a LAN switch.
At other computers there is at least one other switch enroute.
It seems all of the computers have internet access otherwise.
It appears that site firewall settings have no affect on this.
Very puzzling that some computers can access fully and others not.
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Fred MarshallPrincipalAsked:
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Fred MarshallConnect With a Mentor PrincipalAuthor Commented:
Found the solution:
Had changed the MTU on the firewall interfaces at the one site from 1500 to 1472.
Had changed the MTU on one of the computers on the LAN to 1472 also.
All other computers on the LAN likely were defaulting at 1500.
For some reason packets related to the Yahoo Calendar Add and Delete functions didn't like the result.
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