For the record : How many times did Rome sack Ctesiphon ?

[Epictetus]

So, does someone know a source that mentions Galerius capturing Ctesiphon?

Interesting. This seems to be very commonly asserted. I just checked Michael Grant and he says so too, but doesn’t give a source. So I checked CAH and found this:

In 297, after an initial defeat at Callinicum, Galerius gained a spectacular victory over Narses and captured Ctesiphon… These victories, depicted on the triumphal arch erected at Thessalonica in honour of Galerius…(47)

Note 47: Millar, Near East 174-222

Cambridge Ancient History, The Crisis of Empire, A.D. 193-337 page 229

So perhaps this comes from archaeology and not from a literary source?