I have probably the same issue:
I select an item for a booking, select the date, then I press "book now" and the window with the booking data opens. I press "continue reservation" and make an additional reservation, pressing "book now" once again. Finally I proceed to the checkout. In the Checkout area are several opportunities to cancel the progress. Whichever I choose, I'm always getting to the "Manage reservation" area. But I cannot exit this "manage reservation" window. All cancel/delet/reset buttons are without function.

I think there is a major missunderstandig:
I pointed to a missing feature AND a bug. The answer you gave is pretty much the same you can find a lot of threads:
Yes we know about it, but if you want to have it, you have to pay the support.

Thats not a way to keep the customers...

Well, this is not a solution!
I want the customer to select the amounts of week he wants to rent the object. Minimum renting time is one week, but if he likes, he can rent it for 5 or 27 or 46 weeks. Its not suitable to create hundreds of reservation types. Also he should get some discounts in regard of the number of weeks he is renting the object.

Additional problem: If you have a reservation type with fixed amount of days. The frontpage still shows the button with the request for the "select end of booking period". Selecting this causes an error massage. This is confusing or the customer!

Same! showed up with upgrade
BookIt Vers 1.5.2
Joomla 2.5.7
PHP 5.3.18

But: even if the php-version would be an older one, you can't just upgrade the requierments for the system without prior notice!!!!

Here I got the same issue. Thanks for posting the code, it worked.

Nevertheless: I bought a system which I expected to work free of errors. Other companies in the software industrie have a point: "known issues" I you know already the solution for certain errors, you should publish them to keep customers away from going crazy be searching for the error.