This thread is 5 months old, but I'm going to bump it with relevant information.
Using ReShade 0.19.0:
MH 2015 x64 will not connect to the server at the login screen. It will give an error about not being able to reach the authentication server.
MH 2015 x86 will connect just fine.
In either of the above two situations, ReShade correctly applies its effects.
However, as swaggerwocky pointed out, using the older SweetFX Boulotaur2024 x64 version with MH 2015 x64, I am able to connect to the login server without any issue. So, ReShade's x64 DLL is causing something to happen.
If anyone wants to test it out, MH 2015 is F2P. The option to switch between the 32-bit/64-bit client is in the launcher and it will create two separate folders. one with each version of the executable file.
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