java.lang.AssertionError: java.lang.reflect.InaccessibleObjectException: Unable to make protected final java.lang.Class java.lang.ClassLoader.defineClass(java.lang.String,byte[],int,int) throws java.lang.ClassFormatError accessible: module java.base does not "opens java.lang" to unnamed module @11e67f7a
at li.cil.sedna.riscv.R5CPUGenerator.generateClass(R5CPUGenerator.java:110)
at li.cil.sedna.riscv.R5CPUGenerator.(R5CPUGenerator.java:26)
at li.cil.sedna.riscv.R5CPU.create(R5CPU.java:13)
at li.cil.sedna.riscv.R5CPU.create(R5CPU.java:17)
at li.cil.sedna.riscv.R5Board.(R5Board.java:58)
at li.cil.oc2.common.vm.AbstractVirtualMachine.(AbstractVirtualMachine.java:71)
at li.cil.oc2.common.tileentity.ComputerTileEntity$ComputerVirtualMachine.(ComputerTileEntity.java:389)
at li.cil.oc2.common.tileentity.ComputerTileEntity$ComputerVirtualMachine.(ComputerTileEntity.java:385)
at li.cil.oc2.common.tileentity.ComputerTileEntity.(ComputerTileEntity.java:77)
at net.minecraft.tileentity.TileEntityType.func_200968_a(TileEntityType.java:89)
at li.cil.oc2.common.block.ComputerBlock.createTileEntity(ComputerBlock.java:98)

I think this may be an issue with your Java installation, it would be helpful if you post the full crash log from the launcher or from the .minecraft directory if it's not in the launcher anymore which provides more info about your system.

Yes, please provide the full log or at least system information, i.e. Java version and flavor used etc.

I lost the log

Then I'm afraid there's not much I can do, sorry. If it happens again, please create a new ticket.