The error messages clearly state that the equation system "simulation.nonlinear[1]" could not be solved. This is a nonlinear algebraic equation system solved during the computation of the ODE RHS. Another variable calculated earlier in the computational causality might be wrong, but this is the first part of the equation system which could not be solved.

You can activate "Simulation Setup/Translation/Model translation/Generate listing of translated Modelica code in dsmodel.mof" to get the translated model. In the dsmodel.mof file you can find a description of the equation system "simulation.nonlinear[1]".

If "Simulation Setup/Debug/Nonlinear solver diagnostics/Details" is activated, you should get a simulation log message "...plotArray(Amat[:,1],Amat[:,2],-1);...". If you copy/paste this to the Dymola command window, you will get a plot of the residual. You should see that the residual function does not reach zero.

Either this is a structural modeling error which is not detected by the compiler, or the model cannot be solved for the given boundary conditions.

You should analyze the "simulation.nonlinear[1]" in the dsmodel.mof and try to get rid of the nonlinear equation system by changing the equations. Often idealized physical dependencies lead to algebraic equation systems. If you cannot do that, you must make sure that it is always possible to solve it.

You can model describe systems with capacities, resistors, and inductors (e.g. Bond Graph Modelling, see "Continuous System Modelling" by Cellier). This is what you described above as "flow" (i.e. resistor) and "volume" (i.e. capacity). There should be no nonlinear equation systems due to the composition of these base components if you stick to the design rules.

You can also describe the system using linear graph modeling. I.e. using through- and across-variables. You might have to clarify how to extend these approaches using the stream connector concept of Modelica. However, the Modelica community usually implements the first approach, and Dymola provides proper debugging features based on the first approach.

It is a design choice how you implement your approach in Modelica component models and connectors. Using a/b connectors you will be able to enforce certain usage patterns.