If you have tried to set block limits for modded blocks, its likely you have not got the correct block id.
To fix it:
1) Make sure the server is stopped
2) Go into the file explorer and right click edit the SpaceEngineers-Dedicated.cfg file
3) Scroll down untl you find where the block limits are, remove the ones that you have added, you must remove the whole code segment, which will look similar too:
4) Once done, click save. Then go back to the game configuration and you should then be able to save normally.
To get the correct modded block ID, please ask the mod developer, or if you have placed the block, you may also be able to find it in the sandbox.sbc file for you map, to check here:
1) Go into the file explorer, then the saves folder then in your map folder where the modded block is placed.
2) Find the sandbox.sbc file and right click > Edit. If you have a large map, you will need to download it and edit it on your computer, then upload after.
3) Scroll down and look for code lines similar too:
<DefinitionId Type="MyObjectBuilder_CubeBlock" Subtype="LargeBlockArmorBlock" ></DefinitionId>
4) The block ID is what is listed after Subtype, for the above example, it would be: LargeBlockArmorBlock
You would place that in the block limit area in the game configuration.
If you would like further assistance, please feel free to contact our support team here.