This may be a long shot answer. Our org found that the number of sub-projects that can be linked to a master is based on the size of the individual project schedule names. Project appears to concatenate the names together into one field and if the total length of all the names exceeds the field length, you start getting open errors. Basically, the longer the project schedule names, the less of them you can link into a master. You can confirm or rule this out rather simply. Either create a master and add each schedule to it one by one, opening and closing with each new sub-project…or try disconnecting sub-projects one by one from the master and opening/closing after each removal. If it starts working fine with only 8 sub-projects for example, then the problem might have been exceeding the field size that stores all the sub-project names. Like I said, it’s a long shot, but our org found we could realistically link between 5 and 10 sub-projects before we started getting errors.