unable to view subscribers list only

Hello there,
I have the pro version installed and everything working just fine, except that I can’t access to the subscribers lists. I am getting an HTTP 500 error.

Don’t have a clue about how long things are like this, since I send the newsletter weekly but haven’t been adding subscribers for almost two weeks now.
Now that I needed to I can’t get access.

I’ve already erased and replaced both pro and regular plugins through FTP but nothing changed.
The plugin error log stopped registering anything on the 31 of December.

Have you got any idea of what it may be and what can I do?

Appreciate all the help you can give.
Thanks,
Sofia