Definitely looks like I bug. I ran into the same issue. Like the error says, CF is failing to convert a BigDecimal to a string in order to use it in the replace function. This should work for you.

<cfset SubjectText = Replace(SubjectText,"%Request.RequestID%",javaCast('String',Request.RequestID),"All")>

For reference, here is an example that fails. In my case, the real value for the theBigDecimal was being generated by selecting an integer column from a simple query against Oracle. As you can see below, the javacast from bigdecimal to string works, but whatever it is doing in the replace function fails. I'll see if I can find were to file the bug.

<cfscript>

theBigDecimal = javaCast("bigdecimal", 123456);

theBigDecimalasString = javaCast("String", theBigDecimal);

newString = replace('This is my text with documentID in it','documentID',theBigDecimalasString);

writeDump('This Works: ' & newString);

writeDump('This Fails:');

newString = replace('This is my text with documentID in it','documentID',theBigDecimal);

</cfscript>

You shouldn't need the pound signs (#) around the variable Request.RequestID, although it appears to work even with them.

Can you give exact examples of what you are using for each variable.

I tried this and dont get any issues with it. - See Example http://trycf.com/gist/610a1e84aee0c490f4a2208296f890ee/acf2016?theme=monokai