We're not aware of Error 68 thrown on E34.

It may not be well documented. However later e34 520i was made with M52B20 engine, same error codes as other chassis such as the popular E36 M52 (B20/B25/B28) so best to look there for clues, as @Shogun did.

Here are all the possibilities:

>96 Error code - Description

2A68 VT-Output-stage (bank2)

2868 VVT-system overload bank2)

2E68 Knocking sensor signal 1
2768 Phase generator positioning failure
2768 Camshaft sensor inlet: phase position
M4x: 4cyl Control valve, Injector 1
68 VANOS, faulty exhaust reference value
68 Fault code memory error

68 Exhaust camshaft VANOS, over-advanced or system performance

M52 is a rock-solid engine, with age and wear things like cam sensors, oxygen sensors and injectors should all be changed/checked good. Make sure FIRST your battery is capable of maintaining >12.0V with a load on it. Old batteries can cause voltage drops that generate errors- but usually airbag ones. Also can cause limp mode, no start, and many other strange things- to the point where the sane end up believing in ghosts.

Whilst running system voltage should be 13.7-14.0V.