I’m not an avid ModelingToolkit user but having had a quick look it seems ODESystems takes a keyword name, that in the newer versions are required (throwing this error if it is not supplied) while in version 5.25.0 it was not required.
So it should probably work if you either supply the keyword name to your system or, as they suggest, use the @named macro (which pretty much does the same thing in the background, just uses your variable name as the supplied name if I understood correct).
using ModelingToolkit
...
eqs = [...]
# First option
mysystem = ODESystem(eqs, name="mysystem")
# Second option
@named mysystem = ODESystem(eqs)
Hope that helps!