Just tried it myself: Installed the new Laz2.0.10/FPC3.2.0 (Win10 / 32 bit) as secondary installation and tried to install rxnew via OPM (checked only the rxnew and rxtools packages in the RX node of OPM - rxtools is required by rxnew). Installation of rxnew aborts with error "cannot be installed", rxtools succeeds. Did manual install of rxnew later - no problems now.
I don't know what's wrong here. Does OPM really fail to detect that rxtools is required by rxnew? (*)
But anyway: I did not experience the FCL error that you mention. Can you try a clean recompilation of the IDE? (Tools > Configure Build Lazarus > in the "clean-up" box check "Clean all" and "Switch after building to automatically" > Build.)
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(*)
[EDIT] Deleted the Laz Config folder and did a clean rebuild of the IDE to reset to the state after installation. Then I repeated the rxnew installtion. But this time I only checked "rxnew" in OPM. Now I was asked whether the packages "rxtools" should be installed too. After confirmation the installation was executed without any issues.