This was an issue in hub 7.92HF6
Hub hotfix Hub 7.92HF6 tunnel connections fail wit - CA Knowledge
Seems like maybe the same thing?
just to expand on the error message, do see that in the tunel client hub.log:
hub:  error:0x14082174:SSL routines:ssl3_check_cert_and_algorithm:dh key too small
If so, I would open a support case as we need to further investigate, it is probably related to an updated OpenSSL component in that build.
CA Tech Support
Yes, tunnel client hub log.
And yes, case is already open.
If it's like the last two times this happened, it's because the wrong OpenSSL library was compiled in.
There's some value in being predictable.