MineOS uses the existing credentials of your server (shadow passwords); In practical terms, this means any of the user/password combinations you can use to log into the command line (or SFTP) will be the same combination you use in MineOS.
The usernames you listed, "admin" or "tc"--since they don't exist on your Ubuntu server--will not permit login into the MineOS webui.
The other username listed "mc" is a common login for users who installed via the MineOS Turnkey ISO, rather than the installation process you did atop an existing Ubuntu installation. In this case, the loginname was created by the install process and prompted the user to set the password.
In short, create a new user using normal Ubuntu user-creation steps and you should be set.
The update notes you are looking for are not usually present on the tumblr, as you noted. There are two places, however, where you can find the information you seek.
Official Mojang blog
They can, however be found in the official Mojang blog on their main site. The older posts can be hard to find though, but should be able to find them with some googling, I suggest searching for
site:mojang.com -site:bugs.mojang.com minecraft [version]
to limit the results to actual results. Note that these posts are often updated, even changing the title in some cases, such as the one for 1.8.2.
Sadly, the information about bugfixes is often scattered among multiple posts for pre-releases as well as the main post, making them hard to follow. They also seem to be incomplete.
The wiki's version history
A much more convenient way of browsing the changes is offered by the wiki, in the Version History article, neatly sorted by major and minor version. There is an extra page for development versions as well, if you want to check out the snapshots or pre-releases.
The downside is that this information is user-curated and takes some time to be gathered, especially for non-obvious changes, such as the blacklisting of player head textures not hosted on a mojang server in 1.8.4.
Best Answer
In your Minecraft data folder (by default
%appdata%/.minecrafton Windows and~/.minecrafton Linux) there is a folder calledlogs. It contains the filelatest.logfor your most recent launch of Minecraft and a lot of.gzarchives containing log files of previous launches. In one of those should be the chat log that you want.If you want to search through all of them, you can copy the archives into a different folder, unpack them all there and use for example Notepad++ to search through all files in that folder.