Hello
I have submitted a ESR o the IBM support pages, but may be some one had the same issue and give me an advice what went wrong.
Here is the text I posted in the ESR:

Problem title
TC Plan does not show
.
Problem description
In Team Concert I defined a release backlog with a iteration per
month. Each month has stories and tasks as work items - some are new -
some done - few have change sets.
First everything displayed fine, then first the "Mai" plan did not load
anymore , then the June did not display any more. The screen shows
"loading" forever.
In the web client, everything shows perfectly fine, in the relase
backlog plan (e.g. in view mode "iteration"), the iterations and the tasks in the iterations display fine as well , just opening the plan of a sprint does not load
at all. All the work items can be opened correctly.
It might be related (although I'm not sure) ,that things go wrong, when
the client machine ( a laptop) goes into the "standby mode" by closing
the lid or if the battery is down.
Some other plan (e.g. July) still open properly as well.
It's the standard client accessing the Express Server version with a
DERBY database.
Restart of the server and/or the client does not help.
All services in the server admin page show good status.
Best Regards, Roland