Seronatus is a figure known only from the letters of Sidonius Apollinaris (2.1, 5.13), both of which offer a very one-sided and critical portrait. He was some kind of high official in Gaul, possibly the vicarius of Aquitanica (so Goffart 1980: 246). In 475 he was accused of high treason by the provincials and executed. He had dealings with both the Roman and Gothic authorities in Gaul and was apparently involved in tax collection.
Sidonius mentions in 5.13.3 that Seronatus treated prisoners badly. However, he does not mention or allude to any specific case involving a cleric. It is possible that Sidonius is exaggerating, and that by saying "he throws clerics in prison" he means to say that Seronatus, as the person responsible for enforcing the law, was generally unreliable and often acted unjustly and violently.
The passage is widely commented on in the secondary literature because of the mention of "hospes" (are they barbarian settlers housed in Roman villas?) and the laws of Theoderic. On the hospes, see Goffart 1980: 245-47, Harries 1996:40, Wolfram 2009: 226; Mratschek 2020: 232. On the laws and whether the mention refers to the Breviary of Alaric: Harries 1994: 126.