I've configured ha-dockermon with a 64 character password and couldn't authenticate afterwards:

http:
  username: username
  password: Jn8NY8Uq2YkImt3peuQhC8YNX3gvf3CjRAMjZJeZH4FOK6FjsZoXNGxeBfzktSGj

(Just a sample password, don't worry...)

Any requests responded with a 401 Unauthorized header, both from Home Assistant and Curl.

Workaround: Shortening the password to 28 characters in my case allows the request to go through and everything works perfectly.

Strangely I can't see any limitation of password length in the source code of ha-dockermon, neither any indication of it in the used library express-basic-auth.