@Axel Vogt I now notice that something must have gone wrong with the copying and pasting in the original problem, as in fact I am dealing with 6 equations rather than 5.

Maple now gives me one solution to the problem that matches the assumptions. This is also the solution that I already had (by just looking at the equations). So I have solved my problem now :)

Thanks for taking the time to look at the problem though. I appreciate it!

@Adri van der Meer Thank you very much for your help. I discovered that my 'useassumptions' command in 'solve' caused this weird error message. Your piece of code is definitely clearer than mine, so I am gonna use that in my calculation.