What makes you think that this is related to MonoBook and not the MediaWiki-extensions-Gadgets extension?
Also see https://www.mediawiki.org/wiki/Manual:Errors_and_symptoms#You_see_a_Blank_Page

Judging by the error message and line numbers, you're trying to use a REL1_28 (MediaWiki 1.28-compatible) version of Gadgets on MediaWiki 1.32.0 (in REL1_28, line 211 of /extensions/Gadgets/GadgetsHooks.php references the WrappedString class and L23 of the same file shows that unlike in more current releases, the WrappedString class is expected to exist in its own namespace, as opposed to in REL1_32 and master, where it exists in the "Wikimedia" namespace; see, for example, L24 and L222 of the REL1_32 version of Gadgets).

Try upgrading your copy of Gadgets to its REL1_32 version. You should be able to do this via Special:ExtensionDistributor/Gadgets on MediaWiki.org, for example. You'll also want to make sure that any and all other extensions are also using a REL1_32 version.

@Aklapper: I think it is related to Monobook because it works in Vector. I usually have vector disabled on this wiki, but enabled it just to test. Config change to vector, works. Gadget Hotcat enabled, works. Config change to Monobook, crashes. Disable gadget in Localsettings, works again. Disable hotcat in Gadget settings, and then enable monobook again, works.