tbbttbbt123 - 2015-07-04

So, I don't have the problem any longer, but since I created this bug report, I would like to explain what I did to fix it.
Before I created this bug, I went to the Filter list and got these lines:

--- Filters list ---
WAV Writer - {F3AFF1C3-ABBB-41F9-9521-988881D9D640}
AC3Filter - {A753A1EC-973E-4718-AF8E-A3F554D45C44} - C:\Program Files\AC3Filter\ac3filter.ax (2.6.0.0)
Haali Media Splitter - {55DA30FC-F16B-49FC-BAA5-AE59FC65F82D} - C:\Program Files\Haali\MatroskaSplitter\splitter.ax (1.13.138.14)

So, normally that's fine. But if you're wondering how I was able to get the list, eventhough the problem is resolved for me, the answer is simple: The list didn't change (except for maybe the version if AC3Filter).

Somehow - don't ask me how - AC3Filter.ax must have gotten on my computer with another application. Anyway - downloaded and installed the AC3 Filter from the codec page in the documentation - and boom: it works. Now don't ask me, why it used to work and all of a sudden stopped working - I don't know! But now it works, and that's the important thing.