Sometimes after copying an existing SSRS report or even just a part of its design, parser starts encountering strange errors. For example:
Unidentifiable substring 'Value' in expression. the parser reported error message
There must be something wrong in the mechanism responsible for copying text boxes. In most case, it replaces simple function names with predicates like Microsoft.Value etc. In my case it turned out to be even worse: some bug in strings concatenation; no matter what.
The problem here is that you have no hint about the object name, where such a non-conform string was added. You can try your textboxes one by one by checking all their properties with functions inside: visibility, font, border etc, even labels for its placeholder! And it can become a nightmare when your report contains dozens of them.
Fortunately there is a some workaround. You can open XML files for both of the original report and your copy of it in Notepad++ and run Compare over them (Install this Compare plugin).
Then scroll down to the first bad guy: a couple of strings above you will see its name.
Go back to the editor, find this textbox.
Now check its corrupted property function expressions and fix it.