LTSpice (and all other flavours of spice) solves generic circuits using a honkin' great matrix of nodal Vs and branch Is, so to have any term go to zero or infinity generally upsets its ability to invert and thus solve the matrix. That's why it often misbehaves when we have an open circuit, but works when we put 100 Mohm across it.
If you were writing your own simulator, you could identify an open circuit, and change the circuit topology to exclude consideration of any path between the open circuit nodes. However, no versions of Spice that I know of have this facility.
You have several options.
- If the relay doesn't switch during your simulation - make two circuit diagrams, and solve each separately.
- Write your own simulator.
- Find out what the largest relay Roff is for which the simulator will still converge, and what the smallest Roff is which you can tolerate in your model. Note that in real life, Roff is never infinite, so your capacitor will run down in a week or a day. Does it matter if it runs down in a minute or a second, can your analysis allow for this? If the two ranges overlap, use an Roff in that range. If not, see one of the first two options, or work on your analysis so that you can work around a finite current.