Can only login as Guest, cannot read encrypted USB

[Dan_Bishop]

There was a previous post with request for help on a similar issue, but there is a significant difference:
The previous post stated that the owner changed something about user names, etc. In my case I changed nothing.
In my case, the only thing that changed was that normally I work from home through my private cell phone network access. When the incident happened I was travelling in the UK and using various public free Wi FIs.
As stated in the title, in my case there is also the problem that I cannot access now any of the data in my encrypted USB.
I am a naive but intuitive Mint user, and I guess that this is a malicious incident.
Therefore, as soon as I get home, I plan on re-installing Mint from scratch and changing passwords.
As a routine, I don't store data in my computer, and I would not lose much time by re-installing Mint,
Looking to hear what the non-intutitive experts say...

[Mute Ant]

If you imagine your machine had been stolen, there could easily be a post asking for help, on the lines of...
'Can only login as Guest, cannot read encrypted USB'
...because Linux won't unlock data unless you can prove you are the owner. It's not your machine any more, really.

[Dan_Bishop]

The encrypted data was on a disk on key. If Mint allows me to use the computer as a Guest, why shouldn't a guest be allowed to use an encrypted disk on key, for which the Guest has the password?

[Dan_Bishop]

If you imagine your machine had been stolen, there could easily be a post asking for help, on the lines of...
'Can only login as Guest, cannot read encrypted USB'
...because Linux won't unlock data unless you can prove you are the owner. It's not your machine any more, really.

Your response supports my notion, that the event was a malicious one. The question then are:
1) Is it a known bug in Mint, which permitted such incident?
As noted in my original posting, there was another post on the subject in this forum, where the one, who posted, was under the impression that the incident was the outcome of some changes he made to the setup.
2) Any advice on how to protect oneself against such attack?
As noted in the original post, I did not suffer any data loss, since I don't keep data on my laptop. In response to the incident I re-installed Mint and case was over. However, it undermined my confidence in Mint.