Could be something as daft as PAPI's not working or something like that? The approved ops manual will lay down the criteria for airfield use
Here in the US, there are very many instrument approaches that require the VGSI (PAPI, VASI, ...) to be operating in order for night operations. This is particularly the case anytime the visual segment is not clear of unlit obstacles on a 20 to 1 slope. Without the VGSI, you can't see and avoid the obstacles.
What suprised me is that they did not fly 5 mins up the road to Kerkira LGKR, or drop into Athens LGAV which has an ILS and all the goodies (but probably a big landing fee).
LGTS is quite a trip, right across the Greek mainland.
The diversion may have to do with standing agreements between airline and handling agents. Indeed a logical diversion would have been Kerkira LGKR.
Monday operation schedule: MON 0325-0845 1330-1930z
METAR LGKF 131750Z 33014KT 9999 FEW015TCU SCT018 19/12 Q1008=
METAR LGKF 131850Z 32017KT 9999 FEW015TCU SCT018 18/13 Q1008=
Official night (SS+30) on May 13th in Greece was on 1755z
Only NOTAM related to lighting:
B0250/13 - LGKF: LIGHTING OF TWYS A AND B OPERATES EVERY SECOND LAMP. 31 MAR 15:23 2013
UNTIL 30 SEP 23:59 2013. CREATED: 31 MAR 16:22 2013
So there's nothing to indicate it was a (documented) lack of lighting.
Maybe it was indeed an unserviceable PAPI or something, who knows.
[edited for text formatting]