Hi Lasse,
it's not a problem with the RFC connection: the RFC connection seems to be fine. These error messages are created by the ABAP side (and only transported back to the RFC client via the existing connection). So you should contact the support team responsible for the ABAP code on backend side for help on how to avoid the errors.
The first error looks like the ABAP Function Module dumped because of a programming error (accessing a field with invalid offset). Unfortunately the name of the FM cannot be seen in the error, but there should be more details about the dump in ST22.
In the second case the FM "/BODS/RFC_READ_TABLE" gets called and returns the ABAP Exception "DATA_BUFFER_EXCEEDED". The responsible OSS component for that FM is BW-SYS. This, however, does not look like a programming error. Problem could simply be that the ABAP side runs out of memory?! That would explain, why it sometimes works and sometimes fails: it could depend on what other workload is going on on the App Server at the same time. If the system is pretty much idle, the call goes through, but if other memory intensive reports are running at the same time, it does not get enough memory and returns the above error?!
Best Regards, Ulrich