Thank you John,
The problem of a IVAP disconnection every 45 seconds or about is recurring, I searched for it on google and many users report that thing since years, but I did not find a fix.
I have suspected this problem was FSUIPC-related because two problems appeared at one time and both of them are related to communications between FSX and external software, which is one of the FSUIPC functionalities. I looked at what was changed on my PC since the last successful FSX flight and saw a Windows 8.1 update concerning NET Framework applied the 3rd of January, so I restored the previous PC state and this fixed both problems.
I apologize for the misunderstanding... I should have written "Perhaps a FSUIPC update is necessary for the FSX version (4.x)", I forgot to write the word "update"
I have got (regularly purchased at simmarket and registered) the latest version of FSUIPC4, released in 2018. But now I see the newer FSUIPC4 version for FSX-SE beta, so I will update to this one.
Moreover, I will not install NET Framework updates anymore in future... or will be very careful with them 😉