A bug in WebKit used by Safari 10.1 is causing the problems in multiple libraries used in ripple-lib (bignumber.js, etc...). More information is available here.
We have tested the developer version of Safari (11) which seems to solve the problem. Please update your Safari to version 11.
This issue has been reported to cause the "Watch-only error" for newly created XRP Ledger wallets. Any funds sent to a watch-only address are likely to be lost forever.
It is highly recommended to test whether an XRP Ledger wallet works by executing a transaction with it after it has been activated with 20 XRP. Setting a trust line, opening an order, exchanging funds and sending a transaction will not be possible if there is a mismatching key pair.