TYPO3 Core

Can you disable the compression (e.g. with BE|debug in Install Tool)? That should give you a better reable version and a hint where this bug really is.
Mind that BE|debug will break the CSS in IE.

[BE][debug] results in exactly the same error, only filename and path changes from

typo3conf/compressor/prototype-95ab4563f841d139fe8705d1d5a55e8a.js

typo3/contrib/prototype/prototype.js

I just copied typo3/js/clearcachemenu.js from 4.5.22 to the 4.5.23 sources and it worked again for IE8, but results in blank page in IE9 (as expected)

i just did a clean install (vm with win7, xampp and typo3_src+dummy-4.5.23). IE8 version is 8.0.7600.16385 and even with this setup cached files remain in typo3conf after choosing "clear all caches".

The environment I tested in first was with strict antivirus (which I can't turn off) and stuff so I decided to do that clean install (other machine without antivirus and stuff, clean windows).

Patch set 1 for branch TYPO3_6-0 has been pushed to the review server.
It is available at https://review.typo3.org/18795

Patch set 1 for branch TYPO3_4-7 has been pushed to the review server.
It is available at https://review.typo3.org/18796

Patch set 1 for branch TYPO3_4-6 has been pushed to the review server.
It is available at https://review.typo3.org/18797

Patch set 1 for branch TYPO3_4-5 has been pushed to the review server.
It is available at https://review.typo3.org/18798

The patched line

event = event || window.event;

Only the changelog of TYPO3 6.1 includes "[BUGFIX] Clearing cache from toolbar fails in IE8".

In (at least) Internet Explorer 9 and 10 clearing the cache currently does not work (X-UA-Compatible" content="IE=8" is set). In 4.5 nodeType-Javascript-Error, in 4.7 white page.

As the change is already in the current version masters you could either checkout them or wait until the next release announcement.