What is the trip current for the RCD? If 30mA then the RCD problem is
easy to explain.

Computer power supplies tend to be of the switch-mode type rather than
linear. Anyway the point is that switch-mode power supplies generally
have a relatively high earth leakage current (typically 3-5mA). This
means that it isn't at all surprising that you can trip the RCD
occasionally as you must have 2 or 3 power supplies per workstation if
you include printers, fax, monitors etc.

To fix this you need not to have an RCD seeing the combined leakage
current, i.e. get rid of the RCD incomer. Alternatively you may be able
to swap it for an RCD with a higher trip current to avoid nuisance
trips (100mA).

Bear in mind that this will then mean none of your circuits is
protected by a 30mA RCD and so additional RCD protection is required
for any circuits where equipment may be used outside. This can be fixed
by using RCBOs for other socket circuits.

As to why the MCB trips, no obvious reason, should only be overcurrent.
I'd either test all the power supplies or at least try to identify
whether a particular psu is causing the problem by switching them one
one at a time (with all the others disconnected). Testing is better.

Fash