I had this problem as well - started in the past month or so.
Long story short - I ended up uninstalling Rapport which solved this problem for me. (I dont remember installing Rapport but I do remember the Santander online banking website prompting me to install it a few times - I've checked my Santander login since uninstalling and its fine).
Anyhow - the long story about what I tried to fix this problem is below:
1) Tried to enable the Safari Rapport Extensions
I use Chrome as my default browser - but the web page that pops up refers to a Safari extension. So I checked my Safari browser preferences and sure enough there were 2 Rapport extensions that were not enabled. So I enabled them and restarted my macbook but still the Chrome page popped up.
2) Tried to uninstall the Safari Rapport Extensions
In the Safari Preferences / Extensions page I clicked on uninstall on the extensions. However, it told me you cant uninstall the extensions - you have to uninstall the whole Rapport application (see point 4 below).
3) Tried to install Chrome Rapport Extensions
I tried locating the Rapport extensions in the Chrome Extension store but couldnt find them. Tried the IBM Rapport website and couldnt find a link to them. So I gave up on that option.
4) Tried to uninstall the Rapport application
So decided to go for the ultimate option and just get rid of the Rapport application - which remember I dont recall actually installing in the first place and secondly doesnt seem to impact my use of online banking.
Uninstalling Rapport isnt as easy as I thought. It is not installed as a standard mac osx application. You have to find/download the Rapport installation dmg, run it, select the Unintsall option.
So finally the page popup is gone on a reboot and I can still use my online banking as before.
Dont think much of this Rapport application.
Hope this helps.