Argh! I am in deep schtuk!
I thought I would treat myself over the holiday and so some genealogy. Surprise, surprise . . . RM won't load . . . .
I am an iMac user and I am having the same problem as described above. I have read the instructions on how to reinstall but I cannot back up my data as the RM program will not open.
It has been nearly a year since I last used RM and backed up my data. I backed up each time with the RM program option and also saved the data as a .rmgb file on a thumb drive.
And as others have found, the problem seems to have appeared after I installed the new iMac OS (High Sierra).
The last thumb drive backup was January 2017. There may be later data but I am not sure.
What can I do?
First, find your RM files (of all types) and copy them to somewhere safe.
Then, on the assumption that your iMac does not have pure SSD rather than an HDD or a Fusion Drive, follow Renees instructions fully and completely - all the steps.
Then, open RM and change the Folder options to point at a copy of the folders in which you saved the copy of your old RM files.
Then try opening the latest data file - it should open OK - but if not, restore from the most recent backup file.
Of course, the last steps will only work if you have got beyond the problem you have reported, which should be the case if you have followed the instructions to the letter and do not have a SSD only boot disk.