cannot find external module: /usr/local/lsws/modules/cache.so

after the upgrade OpenLiteSpeed to 1.5.10, the cache.so module disappeared?

Same
cannot find external module: /usr/local/lsws/modules/cache.so

I have same issue, only cache.def thereā€¦ And my cache is not working

As above, it is moved to server core.
But there is bug in WebAdmin which will be fixed in the next release. Right now, if you get that issue, you can make a fake cache.so in that location, just in case you want to change the setting of it in webAdmin.
Thanks
David