Again - thanks loads for bearing with us whilst we track down the problem.
There's another thing to try (and yes, we know it's frustrating, but we're getting there).
As we can't replicate the problem ourselves (and I've tried repeatedly) we'd like to ask for some additional information from you. We need the 'click string' (the whole link address) from the link to see if this can give us some additional insight into the mystery black boxes.
If you have time we would really appreciate if you could help - below are some instructions for how to do this.
When the black boxes appear, please could you disconnect from the internet - by turning your phone on to airplane mode.
Then click or tap the black box. This should try and attempt to open in a browser window, but because there's no service it won't load - so you get the error message.
However, the click string will be shown in the URL in the site address box (see image).
Please could you capture this web address and post it on the thread.
Thanks to all who have got this far. We're trying to chase down the problem and really appreciate your help.
M.