I use the function SysFile.SysFileOpen(sFileNamePara, SysFile.ACCESS_MODE.AM_WRITE, pErrorFileOpen); to open and write a file.
Since the update from Codesys V3.5 SP17 to V3.5 SP19 Patch 5 it does not work anymore. I first had SysFile.SysFileOpen(sFileNamePara, SysFile.ACCESS_MODE.AM_APPEND_PLUS, pErrorFileOpen); in both versions.
hFile_Test is a pointer to byte. It gives an exceptional error when dereferencing the hFile_Test pointer.

Edit: I use Codesys on a Raspberry pi

At 19.1 the hFile_test should be RTS_IEC_HANDLE and always open-write - close in one cycle

udi_handle:             RTS_IEC_HANDLE;         //handle-number. Is created by SysFileOpen

udi_handle:=SysFileOpen(sFileName,Sysfile.AM_WRITE_PLUS, ADR(udi_ErrorCode));       
udi_RetValWrite:=SysFileWrite(udi_handle, pRemBuffer, szRemBufSize, ADR(udi_ErrorCode));    
//closing file  
IF udi_RetValWrite < szRemBufSize THEN  ui_Error:=cWriteError;  END_IF//error writing data
udi_ErrorCode_Close:=SysFileClose(udi_handle);

Finally this solved my issue: https://forge.codesys.com/forge/talk/Engineering/thread/3da9ded84e/, and the official documentation https://faq.codesys.com/display/CDSFAQ/SysFile%2C+Persistence+Manager%2C+Datalog+Manager . Add the media folder to the PlaceholderFilePath. Like this:
PlaceholderFilePath.2=/media/pi, $usb$
PlaceholderFilePath.2.View=1
PlaceholderFilePath.2.Volatile=1

But thank you!