You are unable to access K2 application components such as K2 Designer, K2 Workspace or K2 Management. This issue occurs due to the use of HTTP/2 in newer browsers, which has stricter requirements than HTTP/1.1. For more on HTTP/2, you can read the following Microsoft documentation: https://tools.ietf.org/html/rfc7540#page-5
You will see the following errors:
"ERR_SPDY_INADEQUATE_TRANSPORT_SECURITY" when using Chrome.
"NS_ERROR_NET_INADEQUATE_SECURITY" in Firefox.
This often occurs with the use of Windows 2016.
Although not a K2 issue, the errors caused by use of HTTP/2 can prevent usability of K2 components. Thus, this can be a crucial thing to check when coming across these types of errors.
This issue is primarily caused by incorrect ordering of blacklisted cipher suites. To re-order the cipher suites, you can download the IIS Crypto 2.0 tool here: https://www.nartac.com/Products/IISCrypto/Download
Once downloaded, you can refer to the following Microsoft documentation for steps on how to reorder blacklisted cipher suites to the bottom of your list: https://support.microsoft.com/en-us/help/4032720/how-to-deploy-custom-cipher-suite-ordering-in-windows-server-2016
You can find a complete list of blacklisted ciphers here: https://tools.ietf.org/html/rfc7540#appendix-A