Hm, there is no incoming ATH-SKG for A3101 as regularly is the first flight of the day, and it has a stellar on-time performance. Based on last 3 months data from FR24 the last delay (before yours) was on July 29th.

Now what I see that happened on your day is that the aircraft operated an extra rotation ATH-SKG with departure from ATH at 05.00 and arrival at 05.55. That is not a regular flight and it might be related to the fact that the previous day (02/Oct) A3128 from ATH-SKG was cancelled (a/c flew to SKG and returned - maybe weather?).

I suggest you make a claim to A3, if they deny it you can try go with some of the agencies (airhelp etc.) - my experience is that A3 tends to pay in order to avoid legal actions.

Given the current climate, A3 will claim that it was ATC but the original reason for the delay seems to be under the control of A3 and therefore they are liable. If they deny the claim