PHP Fatal error: Uncaught Error: Access to undeclared static property: Smarty::$_CHARSET

I would say go for 2.6.0. I'm beginning to suspect that the older MODX versions from 2.4.0 to 2.5.0 are either corrupted or have problems with newer versions of PHP. I had a serious problem with 2.5.0 recently which went away when I upgraded to 2.6.0.

If things work with 2.6.0, upgrade to 2.6.5 to get the security fix. Then you might want to wait for 2.7.1 when some of the error log issues on deprecated methods will be straightened out.

PS: I've sent you an email on this.