Mollie module, like many other modules, depends on third party libraries. These libraries are part of module zip file. And module contains some specific version of the library. For example, mollie it can contains on guzzle v6.0 while stripe can contain guzzle v7.0.
These libraries are load on demand using autoloader. The first module that is used will load its version of library.
For example, if the stripe module hook is called first, then system will use guzzle v7. If the mollie hook is called first, system will use guzzle v6. What's worse, system can use different library versions for different requests.
So, if the module works on not may depend on existence (and installation order) of other modules in the system. Crazy.
There is nothing tb core can do to prevent this.
We can implement some detection mechanism and warn user if there are two (or more) versions of the same library on classpath. But that's about that.