Since the past few days, some Gmail accounts have not been able to navigate the Gmail OAuth2 process. This does not currently affect all accounts, though it likely will in the near future.
An update is available which fixes this problem. Before being officially announced, I have made this available for testing. Anyone is welcome to install this version, but if you have the particular Gmail issue, please post your results in the following topic (or email support) to let us know:
Gmail OAuth discussion
Download v4.5.3