I managed to get Nuendo into a state where the fader caps were completely transparent, and the default audio clips were also transparent with no audio displayed. Unfortunately I don’t know exactly what I did, but (among other things) I was messing around with track colours in preferences.
I tried a heap of different things to fix the problem, including launching in safe mode, restoring defaults from within preferences, but to no avail.
After a bit of poking around, I discovered a bunch of values in UserPreferences.xml had been trashed (see all values set to 0 below)
<item>
<string name="Group" value="TrackDefaultColorPreferences"/>
<member name="Values">
<int name="audio" value="-7364173"/>
<int name="effect" value="-7364173"/>
<int name="group" value="-7364173"/>
<int name="instrument" value="-7364173"/>
<int name="midi" value="-7364173"/>
<int name="other" value="-7364173"/>
<int name="rewire" value="-7364173"/>
<int name="sampler" value="-7364173"/>
<int name="track-types.audio" value="0"/>
<int name="track-types.effect" value="0"/>
<int name="track-types.group" value="0"/>
<int name="track-types.instrument" value="0"/>
<int name="track-types.midi" value="0"/>
<int name="track-types.other" value="0"/>
<int name="track-types.rewire" value="0"/>
<int name="track-types.sampler" value="0"/>
<int name="track-types.vca" value="0"/>
<int name="vca" value="-7364173"/>
</member>
</item>
<item>
<string name="Group" value="FaderColorPreferences"/>
<member name="Values">
<int name="audio" value="-5052929"/>
<int name="effect" value="-6269205"/>
<int name="group" value="-12539415"/>
<int name="inputs" value="-1689801"/>
<int name="instrument" value="-1710432"/>
<int name="midi" value="-798807"/>
<int name="mixer-fader-caps.audio" value="0"/>
<int name="mixer-fader-caps.effect" value="0"/>
<int name="mixer-fader-caps.group" value="0"/>
<int name="mixer-fader-caps.inputs" value="0"/>
<int name="mixer-fader-caps.instrument" value="0"/>
<int name="mixer-fader-caps.midi" value="0"/>
<int name="mixer-fader-caps.outputs" value="0"/>
<int name="mixer-fader-caps.rewire" value="0"/>
<int name="mixer-fader-caps.sampler" value="0"/>
<int name="mixer-fader-caps.synth" value="0"/>
<int name="mixer-fader-caps.vca" value="0"/>
<int name="outputs" value="-1689801"/>
<int name="rewire" value="-3259162"/>
<int name="sampler" value="-5062657"/>
<int name="synth" value="-5046350"/>
<int name="vca" value="-12785281"/>
</member>
</item>
<item>
<string name="Group" value="RackColorPreferences"/>
<member name="Values">
<int name="devpanels" value="-7951168"/>
<int name="directRouting" value="-14731968"/>
<int name="directRouting-sum" value="-14729152"/>
<int name="eq" value="-14447326"/>
<int name="filter" value="-13582289"/>
<int name="foldback" value="-13647704"/>
<int name="foldback-pre" value="-13647775"/>
<int name="gain" value="-7951168"/>
<int name="hardware" value="-7951168"/>
<int name="insert" value="-13659968"/>
<int name="mixer-racks.devpanels" value="0"/>
<int name="mixer-racks.directRouting" value="0"/>
<int name="mixer-racks.directRouting-sum" value="0"/>
<int name="mixer-racks.eq" value="0"/>
<int name="mixer-racks.filter" value="0"/>
<int name="mixer-racks.foldback" value="0"/>
<int name="mixer-racks.foldback-pre" value="0"/>
<int name="mixer-racks.gain" value="0"/>
<int name="mixer-racks.hardware" value="0"/>
<int name="mixer-racks.insert" value="0"/>
<int name="mixer-racks.insert-pre" value="0"/>
<int name="mixer-racks.pan" value="0"/>
<int name="mixer-racks.quickcon" value="0"/>
<int name="mixer-racks.routing" value="0"/>
<int name="mixer-racks.send" value="0"/>
<int name="mixer-racks.send-pre" value="0"/>
<int name="mixer-racks.strip" value="0"/>
<int name="mixer-racks.vca" value="0"/>
<int name="pan" value="-7951168"/>
<int name="quickcon" value="-10535040"/>
<int name="routing" value="-7951168"/>
<int name="send" value="-13666112"/>
<int name="send-pre" value="-13647680"/>
<int name="strip" value="-4169169"/>
<int name="vca" value="-7951168"/>
</member>
</item>
<item>
<string name="Group" value="StripColorPreferences"/>
<member name="Values">
<int name="Comp" value="-11761728"/>
<int name="Gate" value="-11157410"/>
<int name="Limit" value="-4179655"/>
<int name="Sat" value="-4179655"/>
<int name="Tools" value="-11761728"/>
<int name="mixer-channel-strips.Comp" value="0"/>
<int name="mixer-channel-strips.EQ" value="0"/>
<int name="mixer-channel-strips.Gate" value="0"/>
<int name="mixer-channel-strips.Limit" value="0"/>
<int name="mixer-channel-strips.Sat" value="0"/>
<int name="mixer-channel-strips.Tools" value="0"/>
</member>
</item>
Note: I was able to manually restore these to the default values without destroying the rest of my preferences by backing up UserPreferences.xml, getting Nuendo to generate a new one, then selectively restoring the default values to my preexisting xml.