L2 switching headaches

I'm having a bit of an issue at the moment and can't figure out why it's happening - as someone who recently passed CCNP Switch, this is causing quite a bit of frustration at myself!!

We have a number of switches that are solely on L2 duties, the only L3 instance is for remote access to the device. There are no static routes or default gateways configured.

We're introducing a physically separate management network (currently we NAT the NOC machines to a pesudo management network that these switches sit in to manage them).

I can freely add the management vlan to a statically configured access port and SVI, but as soon as I connect the switch into the management switch all hosts connected to the device lose their connectivity, permanently, it's not Spanning tree (rapid-pvst+), recalculating as far as I can tell.

There are circa 150 vlan's existing on the switch, these are trunked up to a number of ESXi hosts (no pruning on any of the trunks).

For reference there are 4 switches and they are all Dell 2048 devices, they're trunked between each other via their 10G SFP interfaces, but there is no loop so logically they sit in a line.

I've labbed this up in GNS3 albeit with cisco devices as I can't find copies of the Dell software to use in GNS3, and it all works perfectly. I have hosts sending continuous pings across all the switches, and I can add a new SVI and an access port, with no loss in service.

Any help is much appreciated, I feel I'm missing something obvious, but can't figure out what it might be.

I can post some sanitised config if required, but it's pretty basic, bunch of vlans, 1 SVI for the old management and a potential