A x10 probe is a DC resistive attenuator (with HF compensation). If you switch the scope to AC mode then, yes, the scope (pretty well all scopes) insert a DC blocking capacitor before the 1M resistive input (I believe there are a few that do it after, but they are pretty rare).
Without a DC path, the probe will present the full input DC input voltage to the scope input, effectively it becomes a 9M series resistor with a bit of parasitic capacitance. Whether a scope can withstand this depends on the voltage rating of its AC coupling capacitor. 400V 300V sounds too high for most scopes.
For a 400V 300V rail, the correct probe would be a X100 one rather than a X10 (even if they claim to be rated that high) and measure on DC. If you need more resolution for a small AC signal riding on top of a high DC one, then the safest approach is to use your own external capacitor, of known voltage rating, attached to the DC rail (again using the scope in DC mode) to provide a charge / discharge path.
Now for the safety disclaimers.... If the 400V 300V rail has any significant energy behind it, big reservoir capacitors for instance, then you need to make sure that the capacitor is well over-rated on voltage, and is preferably a safety type and make all connections before energising it and ensure that the scope stays DC mode, with probe. If it is high energy (Class II or above) then you need to look elsewhere - scopes are standard probes are Class I devices. If the rail is, in any way, directly related to Mains then you can't attach the ground clip to Neutral or any non-ground voltage. The only safe probing method is a high voltage differential probe.
The above is general information rather than VDS1022(I) specific, but I wouldn't go stuffing 400V 300V DC into it in AC coupled mode, any more than I would DC (that is, DC without the appropriate X100 probe).
I hope this helps.
P.S. Take a look at Dave's "How not to blow up your scope" video.