You’re suddenly receiving a “FATAL ERROR: Couldn’t agree a host key algorithm (available: rsa-sha2-512,rsa-sha2-256)” when trying to ssh into your Ubuntu Linx Server
The Couldn’t agree a key exchange algorithm is expected behavior with older versions of PuTTy or WinSCP.
To resolve this issue, upgrade PuTTy to the latest version via the below links. https://www.chiark.greenend.org.uk/~sgtatham/putty/latest.html . Try using PuTTy or WinSCP from a different computer/laptop if applicable.
All the best of luck. My problem was solved by actioning the above.