To know if there is an issue with memory_limit, you have to check the in PHP error log (ask your web provider to know where it is). If you cannot access this log, try to deactivate the editor gzip compression in the platform configuration (Main settings > Advanced settings tab) to see if it fix the issue.
Be aware that the memory_limit directive also affects the generation of zip archives in documents and assignments tools. You have to configure the maximum available space for those tools accordingly to the memory limit on the server or to disable those features in the platform configuration. In my experience, the memory limit should be at least equal to about 1.5 times the size of the available space in those tools + 16 to 32 Mo for PHP itself.
Be aware also that memory limit will affect the maximum size of the files the platform can handle (importing exercises or SCORM package, uncompressing zip archive on the server...).
Frederic Minne (ZeFredz) - Université catholique de Louvain - Contributor to Claroline