For me, the userlib cleaner solved the issue.
You can download it from https://github.com/cinaq/mendix-userlib-cleaner
Works pretty simple:
mendix-userlib-cleaner.exe into your userlib you want to clean
cmd and press enter. This opens a black terminal window
Double check your Jar files in the userlib. Do you have multiple httpcore or httpclient Jar files?
What I know is, sometimes with java actions Mendix change the variable names by itself, thus leaving us with these kind of compilation errors. I have experienced this with custom java actions. But it is strange that you have it for SAML module.
This means, DelegatedAuthenticationHandler.java within SAML module has httpPost attribute created for a class. But probably renamed to something and all the references could still be httpPost.
If it is possible to open the java file and find this (look in line number 305) try renaming it manually, save the file and run the Mendix application.
See if this fixes the issue.
Also check if there are any latest version with proper fixes for the same