Error “The Citrix SSL Server you have selected is not accepting connections.” shows when trying to launch SSL encrypted ICA session.

1. Make sure the SSL configurations on the Delivery Controller and Linux VDA are consistent.  That is, if you want to use SSL encryption, then you should enable them on both Delivery Controller and Linux VDA. Otherwise, disable them on both Delivery Controller and Linux VDA. Refer to How to check if SSL encryption is enabled correctly section of KB CTX230022:How to troubleshoot SSL Encryption for Linux VDA for detailed instructions.
2. If this issue only happens on Firefox (version 52 or above), then try to download and install Firefox 32 bit ESR version from https://www.mozilla.org/en-US/firefox/organizations/all/. 

Problem Cause

There are multiple possible causes for this issue:

1. The Delivery Controller is configured to enable SSL encryption for ICA sessions while Linux VDA doesn’t. Once the Delivery Controller is configured to enable SSL encryption, the generated .ica file will set SSLEnable to on, as follows:

In this case, you can either disable SSL encryption on the Delivery Controller (if you don’t want to use SSL encryption) or to enable SSL encryption on the Linux VDA 
And Citrix Receiver will try to launch ICA session with SSL, but if Linux VDA doesn’t accept SSL connections, then the ICA session will fail.

2. This error sometimes appear even all the configurations are correct if you’re using Firefox version 52 or above to launch ICA sessions. In this case, ICA sessions can be launched successfully from another web browser (for example, IE) on the same client.