Hi Takahiro-san,
Thank you for the report.
According to the error message, I guess your proxy server is using old version of TLS (such as TLSv1.0) which is obsolete.
Recently Inkdrop has upgraded Electron from 1.x to 3.x which runs on node 10.2, no longer supporting obsolete SSL protocols.
Please check if your proxy server supports newer protocols like TLSv1.2.