Hi,
maybe a way to do so. It is just a quick experimentation, as I use Arena to run MessChess engines, not Fritz.
I suspect the UCI protocol to be the root cause of issues, as it is known to be only very basically implemented in MessChess; most probably incomplete.
I tried to ensure compliancy using polyglot, looks to be working fine - at least at first glance. I didn't play a whole game nor performed many tests. I run Fritz19 and I chose Boris for the demo.
I created a directory for this purpose; where I copied polyglot.exe under the file name Boris.exe - so that polyglot will look for Boris.ini file. This can be duplicated for any number of MessChess engines.
Here is the content of Boris.ini:
Code:
[Polyglot]
EngineName = Boris
EngineDir = .
EngineCommand = ..\Arena\mess.cmd borisa
Log = true
LogFile = polyglot.log
UCI = true
UCIVersion = 2
[Engine]
Of course enabling the log was just an option for my tests.
Setting up the engine went fine: just select the Boris.exe file and hit the parameter button (the name field will be automatically completed afterwards).
Here are the parameters:

I have set the Level to 05 secs and let the Speed "as is" by default.
Of course, don't set any book here, nor at the Fritz GUI level (close your usual Fritz book if any).
Load Boris:

The 128MB displayed is just my current hash setting (no use for MessChess)
And the game can begin:
I hope there is no hidden issue, I didn't test much.
MfG,
Tibono