Initial button when requesting e-signatures not working (using Adobe Acrobat Sign)

When sending documents for electronic signature through Adobe Acrobat Sign online, we are running into a problem where our recipients are unable to complete the initial field. It seems to be stuck; when we click on the 'Initial' button it won't let us add an initial - the usual pop-up doesn't appear and nothing happens. However, it will let us fill out the signature in the same document when we click on the 'Signature' button.

We have tried multiple different templates, brand new documents, cleared the cache on our browsers, sent to different email addresses, checked the permissions and participants set, tried it on our phones, and nothing we've done has worked. 

We've only had this problem today, and all the documents we sent out up until yesterday seem to be working as per usual.