I have already contacted the technical support, but not heard back yet. Since this problem is quite frustrating, and I am relying on Mathematica on a daily basis, I am posting this here in the hope to get a quicker response.
A few weeks ago I got a new laptop (Thinkpad T470s) and installed Ubuntu 18.04 together with Mathematica 11.3. Very quickly did I run into the issue that Mathematica quit the kernel at seemingly random stages during a "Simplify" or larger symbolic expressions, or simply larger symbolic computations (it has happened in the context of "Series", "Simplify" and in several applications of the xTensor package). I then installed Ubuntu 16.04, as this is officially supported by Wolfram, with the exact same issue (same for other ubuntu based distributions).
Some sample code (and I can provide many more examples) that causes Mathematica 11.3 to quit the kernel after a good 5 seconds:
var = {g11n[1] -> g11n1, g11n[2] -> g11n2, g12n[1, 1] -> g12n11,
g12n[2, 1] -> g12n21, g12n[2, 2] -> g12n22, g21n[1] -> g21n1,
g21n[2] -> g21n2, g22n[1, 1] -> g22n11, g22n[2, 1] -> g22n21,
g22n[2, 2] -> g22n22};
Hnew[Ps_] :=
H0n[Ps] +
G/r (f10n[Ps] + L/r^2 Sum[S[i] f11n[i, Ps], {i, 1, 2}] +
1/r^2 Sum[S[i] S[j] f12n[i, j, Ps], {i, 1, 2}, {j, 1, i}]) +
G^2/r^2 (f20n[Ps] + L/r^2 Sum[S[i] f21n[i, Ps], {i, 1, 2}] +
1/r^2 Sum[
S[i] S[j] f22n[i, j, Ps], {i, 1, 2}, {j, 1, i}]) //. {S[1] ->
S1, S[2] -> S2};
reprule = {A ->
g10n + L/r^2 Sum[S[i] g11n[i], {i, 1, 2}] +
1/r^2 Sum[S[i] S[j] g12n[i, j], {i, 1, 2}, {j, 1, i}],
B -> g20n + L/r^2 Sum[S[i] g21n[i], {i, 1, 2}] +
1/r^2 Sum[S[i] S[j] g22n[i, j], {i, 1, 2}, {j, 1, i}],
S[1] -> S1, S[2] -> S2, var} // Flatten;
PSnew = g0 + G/r A + G^2/r^2 B //. reprule;
SeriesCoefficient[Hnew[PSnew], {G, 0, 2}]
Now, for obvious reasons, Mathematica shouldn't crash over this. Since this seems to be a problem on several different Ubuntu versions (of which most are officially supported by Wolfram), I suppose it is due to the new laptop, though, I really cannot think of what it might be (some different type of RAM that Mathematica can't handle for some reason?). So maybe you have an idea for how to attempt to solve the issue. Thanks!