ある日FetchやPushしようとすると怒られるように
couldn't agree a key exchange algorithm
とのこと。
SourceTree2.4.7で認知されたバグで、2.4.8にて修正済みとのこと
SourceTree 2.4.7.0 [20 December 2017]
Known Issues
SRCTREEWIN-8264 v0.70.1-Sourcetree of PuTTY/PLink shipped with Sourcetree 2.4.7.0 is unable to read credentials for username/password based SSH connections. This potentially blocks fetch/pull/push actions to remote hosts. Key pair based SSH and HTTPS connections continue work correctly.
とのこと。さらに
SourceTree 2.4.8.0 [17 January 2018]
Changes
SRCTREEWIN-8264 Updated PuTTY/PLink dependencies to 0.70.2-Sourcetree to fix interactions with SSH urls using username/password credentials.
だそうで、SourceTreeのバージョンアップにて解決⸜( ´ ꒳ ` )⸝