Hi,
well, obviously this seems to be a bit weird. The error is a TmoleX one, but it might be caused by an unexpected Turbomole output underneath. Did the single-point energy for Lu work without issues? With or without symmetry? And did you use an all-electron basis set like x2c-TZVPPall ?
As there seems to be no obvious reason for Lu to fail if the other lanthanides worked, it might be a pure coincidence and the cause is more of a technical issue.
Just guessing: If you visualize many many orbitals, TmoleX might run into memory problems. The reason is that by default TmoleX is only allowed to use a small portion of the memory that is available.
You can change that in the Menu Extras -> Preferences -> Processing. In there you can set the amount of memory TmoleX is allowed to use in the field 'Max. memory for TmoleX and 3DViewer', the unit is MB.
The number left to it shows how much total physical memory (RAM) TmoleX found, so you can set something like 75% of that value to avoid that TmoleX fills your memory completely which would make the usage very slow...
Best Regards, Uwe