Another Person has logged into this account.

Abnormal Disconnection.
Another Person has logged into this account.

should i be worried?

RIP your valuables (items and currency), stolen by whoever got a copy of your login info.

RIP your ability to play for several months. GGG will lock your account as soon as they respond to your email, it will take that to go through account recovery. The entry level support person who will initially respond will quickly get some information; afterwards the work is handed off to a senior person who is catastrophically overworked and was taking 4-6 weeks to respond earlier in this year. (I haven't seen any more recent reports with timelines from anyone recovering a lost/compromised account.)

There has been cases of this error being thrown erroneously - although I understand that to be vanishingly rare.

If you haven't already, you really should take any and all possible steps to secure your account. As you could hypothetically be playing through their standalone client; through Steam; through the Epic Games Store; on your Xbox; or on your PlayStation, nobody will be able to walk you through that process with the information currently available.

But seeing that error message should result in a "drop everything and act now" response from the account owner - as the risk isn't worth taking.

If you got that error hours ago and have not done anything yet... well you probably have your answer by now - one way or the other. But I would still recommend changing passwords; changing email address(es); running at least one full system anti-virus scan if appropriate for your device; and reviewing any third-party tools related to Path of Exile you may use and how you log into them.