Error to create modules in the schematic

I still have version 5.0 and it’s (was?) Fully functional, I don’t use it that much. Lately I am doing a new project and the problem of new modules / symbols came back to me (had already done it some time ago but I don’t remember how I solved it …). I state that, even if it is not regular to do so, when I create a new project I copy / paste the entire folder of the last one created and I go to rename all the files involved: so I do not lose anything and I find everything I need! Now, in the new diagram I have inserted all the components and made all the connections, but the need has arisen to create a new symbol that is not there: I open the symbol editor, new symbol, I choose my library inside the project (and in fact there are all the symbols I need), I create the symbol and save it in this library. EVERYTHING OK? Apparently yes, it also makes me put it in the schematic, connect it etc. but … when I close the diagram and go to reopen it from this error, it opens it anyway but my symbol no longer exists, not even in the library !!!
“Failed to load the symbol library … *** cache.lib. Error: expected single character token in input / source” D: \ … Wiring Diagram SET ARDUINO-cache.lib "line 501 , offset 21

It’s possible that when you edit a schematic symbol with the library editor it’s using an absolute path to one of the other copies, while when you try to load it as a project specific library, KiCad may try to load it from another copy. This scenario is a nightmare for maintenance and you have to be very careful with where you store libraries and from which library you get a symbol.

You can view the library links in: Schematic Editor / Tools / Edit Symbol Library Links (at least, I think this also is in the same place in KiCad V5.x)

Your KiCad is also complaining about “[Project]-cache.lib” file, and this is probably another and unrelated problem, or maybe it’s also caused by file renaming.
KiCad V5.x and it’s dependency on external libraries together with the hack of the “[Project]-cache.lib” is a quite dirty concoction. Just to get rid of that it’s probably worth updating to KiCad V6. KiCad V5.0 is also quite old. Why do you stick with it? Each KiCad license comes with a lifelong guarantee of free upgrades.

You are trying to save your symbols into the cache library. This is an automatic generated lib and should not be modified externally.

Save them into any other library.

Thank you very much that was just the problem, I created a new library and now it goes, even if when opening the file it still gives me this error (but everything works, I hope it is irrelevant)