You have a memory limit of 512 Mb defined in your php.ini, and this seems not enought to calculate the planning for your project.
Maybe your project is quite complex and needs more memory.
Try and increase memory_limit in php.ini.
Or try and split you project so that you can plan sub-projects one by one.

If this occurs for small project, check dependencies between tasks : maybe you have dependency loop that leads to recursive loop in planning calculation.

memory_limit was 1024M, we have increased it to 2048 but it still failing the same way. Actually this is happening just for two projects and not the biggest ones

It's Ok now after clearing the cache