I assume the 1810's are allready interconnected before installation of the 6200?
by default spannig-tree is disabled.
So you create loops in your network when powering up the 6200.
-> disconnect all 1810's before powering up the 6200.
- then connect a single 1810 using a single Fibre (pair).
- see network behaviour (probably all normal)
- connect a second 1810
-> now the netweork will go bezerk again
=> spanning-tree problem configred
(disconnect again)
==>> either enable and configure spanning-tree.
or disconnect the old 1810's uplink before connecting to the 6200.