There may be some problems with access to JSTOR: See below for a summary / suggestions: 

The error occurs when a website moves from being ‘http’ to the new standard ‘https’. Many websites are moving to https through 2017 and 2018. 
The updating process to record each site as a trusted site takes time and involves external service providers. 

The error message can be bypassed in most web browsers by adding an exception or security permission directly from the error message page. 
The security permission will be remembered by most web browsers when returning to the website.

In Chrome select ‘advanced’ and ‘proceed to …’.

In Firefox select ‘advanced’ and an additional dialog window will ask you to ‘confirm security exception’.

In Internet Explorer select ‘details’ and ‘go on to the webpage’. Is it safe to connect to the website?

Firefox says of the error message “This error is telling you that the identification sent to you by the site is actually for another site. While anything you send would be safe from eavesdroppers, the recipient may not be who you think it is.”
Links provided on the catalogue should be assumed to be safe. 

There are two features to look out for if you are concerned.
1) Check that the link is genuinely by looking for after the main website address.
e.g. for JSTOR:
2) Check the error code, which should indicate that the error is for SSL, certificates, or domain.

In Chrome the error code is ERR_CERT_COMMON_NAME_INVALID
In Firefox the error code is SSL_ERROR_BAD_CERT_DOMAIN
In Internet Explorer the error code is DLG_FLAGS_SEC_CERT_CN_INVALID